Keep the vendored DiffPlex internal and drop what Pester does not call

Vendoring put 20 DiffPlex types into Pester's public API. Nothing asked for that, and
it would make the vendored library part of the compatibility surface, so replacing it
or updating it across a breaking change becomes our problem.

Every vendored type is internal now. Pester exposes one method,
Pester.StringDiff.Format, which takes the two strings and returns the part of the
failure message that describes how they differ. StringDiffResult and Compare are
internal, and PesterTests sees them through InternalsVisibleTo.

The files were already modified, so there was no reason to keep code we never call.
pester.patch removes Differ's convenience methods, IDiffer, ISideBySideDiffBuilder,
and the SideBySideDiffBuilder constructors and static helpers that default to
LineChunker. Five files then have nothing referring to them and are not vendored at
all. 19 files became 14, and Pester.dll grows by 22 KB per target framework instead
of 24 KB.

WordChunker and DelimiterChunker stay. SideBySideDiffBuilder uses them for the word
level sub pieces on a changed line, which we do not render yet, and highlighting the
changed part inside a line is the obvious next thing to do to this message. Cutting
them means editing the core of SideBySideDiffBuilder rather than deleting whole
members.

Deleting code is what breaks a vendored copy, because a hand-trimmed one diverges
quietly and nobody can tell what was changed on purpose. So the removals are kept as
a patch, not applied by hand. Update-VendoredDiffPlex.ps1 rebuilds the whole thing
from upstream, applies the namespace rewrite, the internal rewrite and the header,
then applies pester.patch, and -Verify compares that against what is on disk. Running
it against a newer DiffPlex either works or git says which hunk failed. -Regenerate
rewrites the patch after a deliberate hand edit.

🤖

Author: nohwnd

src/csharp/Pester/DiffPlex/Chunkers/CharacterChunker.cs

@@ -1,13 +1,13 @@
 // Vendored from DiffPlex 1.9.0, https://github.com/mmanela/diffplex
 // Copyright (c) Matthew Manela. Licensed under the Apache License, Version 2.0.
 // See LICENSE.txt and VENDORING.md in this folder.
-// Modified by the Pester Team: the namespace is Pester.DiffPlex instead of DiffPlex.
+// Modified by the Pester Team, see VENDORING.md. Rebuild with Update-VendoredDiffPlex.ps1.
 using System;
 using System.Collections.Generic;
 
 namespace Pester.DiffPlex.Chunkers
 {
-    public class DelimiterChunker : IChunker
+    internal class DelimiterChunker : IChunker
     {
         private readonly char[] delimiters;
 

src/csharp/Pester/DiffPlex/Chunkers/CustomFunctionChunker.cs

@@ -1,12 +1,12 @@
 // Vendored from DiffPlex 1.9.0, https://github.com/mmanela/diffplex
 // Copyright (c) Matthew Manela. Licensed under the Apache License, Version 2.0.
 // See LICENSE.txt and VENDORING.md in this folder.
-// Modified by the Pester Team: the namespace is Pester.DiffPlex instead of DiffPlex.
+// Modified by the Pester Team, see VENDORING.md. Rebuild with Update-VendoredDiffPlex.ps1.
 using System.Collections.Generic;
 
 namespace Pester.DiffPlex.Chunkers
 {
-    public class LineEndingsPreservingChunker:IChunker
+    internal class LineEndingsPreservingChunker:IChunker
     {
         private static readonly string[] EmptyArray = new string[0];
 

src/csharp/Pester/DiffPlex/Chunkers/DelimiterChunker.cs

@@ -1,10 +1,10 @@
 // Vendored from DiffPlex 1.9.0, https://github.com/mmanela/diffplex
 // Copyright (c) Matthew Manela. Licensed under the Apache License, Version 2.0.
 // See LICENSE.txt and VENDORING.md in this folder.
-// Modified by the Pester Team: the namespace is Pester.DiffPlex instead of DiffPlex.
+// Modified by the Pester Team, see VENDORING.md. Rebuild with Update-VendoredDiffPlex.ps1.
 namespace Pester.DiffPlex.Chunkers
 {
-    public class WordChunker:DelimiterChunker
+    internal class WordChunker:DelimiterChunker
     {
         private static char[] WordSeparators { get; } = { ' ', '\t', '.', '(', ')', '{', '}', ',', '!', '?', ';' };
 

src/csharp/Pester/DiffPlex/Chunkers/LineChunker.cs

@@ -1,13 +1,13 @@
 // Vendored from DiffPlex 1.9.0, https://github.com/mmanela/diffplex
 // Copyright (c) Matthew Manela. Licensed under the Apache License, Version 2.0.
 // See LICENSE.txt and VENDORING.md in this folder.
-// Modified by the Pester Team: the namespace is Pester.DiffPlex instead of DiffPlex.
+// Modified by the Pester Team, see VENDORING.md. Rebuild with Update-VendoredDiffPlex.ps1.
 using System.Collections.Generic;
 using System.Linq;
 
 namespace Pester.DiffPlex.DiffBuilder.Model
 {
-    public class DiffPaneModel
+    internal class DiffPaneModel
     {
         public List<DiffPiece> Lines { get; }
 

src/csharp/Pester/DiffPlex/Chunkers/LineEndingsPreservingChunker.cs

@@ -1,13 +1,13 @@
 // Vendored from DiffPlex 1.9.0, https://github.com/mmanela/diffplex
 // Copyright (c) Matthew Manela. Licensed under the Apache License, Version 2.0.
 // See LICENSE.txt and VENDORING.md in this folder.
-// Modified by the Pester Team: the namespace is Pester.DiffPlex instead of DiffPlex.
+// Modified by the Pester Team, see VENDORING.md. Rebuild with Update-VendoredDiffPlex.ps1.
 using System;
 using System.Collections.Generic;
 
 namespace Pester.DiffPlex.DiffBuilder.Model
 {
-    public enum ChangeType
+    internal enum ChangeType
     {
         Unchanged,
         Deleted,
@@ -16,7 +16,7 @@ public enum ChangeType
         Modified
     }
 
-    public class DiffPiece : IEquatable<DiffPiece>
+    internal class DiffPiece : IEquatable<DiffPiece>
     {
         public ChangeType Type { get; set; }
         public int? Position { get; set; }

src/csharp/Pester/DiffPlex/Chunkers/WordChunker.cs

@@ -1,13 +1,13 @@
 // Vendored from DiffPlex 1.9.0, https://github.com/mmanela/diffplex
 // Copyright (c) Matthew Manela. Licensed under the Apache License, Version 2.0.
 // See LICENSE.txt and VENDORING.md in this folder.
-// Modified by the Pester Team: the namespace is Pester.DiffPlex instead of DiffPlex.
+// Modified by the Pester Team, see VENDORING.md. Rebuild with Update-VendoredDiffPlex.ps1.
 namespace Pester.DiffPlex.DiffBuilder.Model
 {
     /// <summary>
     /// A model which represents differences between to texts to be shown side by side
     /// </summary>
-    public class SideBySideDiffModel
+    internal class SideBySideDiffModel
     {
         public DiffPaneModel OldText { get; }
         public DiffPaneModel NewText { get; }