perf: use Stack.Peek to reduce Pop and Push churn (#1708)

Not sure if more changes might help here, perhaps calling
`docsStack.Push(TraverseDocOnExitStackMarker);` later and adding an
`else` clause might help, that or using a goto. Might be easier to
measure an improvement once other changes were made.

Did wonder if `ValueListBuilder` will help due to faster `Push` and
`Pop`.

Co-authored-by: Bela VanderVoort <[email protected]>

Author: TimothyMakkison

Src/CSharpier.Core/DocPrinter/PropagateBreaks.cs

@@ -1,3 +1,4 @@
+using System.Runtime.CompilerServices;
 using CSharpier.Core.DocTypes;
 
 namespace CSharpier.Core.DocPrinter;
@@ -27,6 +28,7 @@ void BreakParentGroup()
             }
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         bool OnEnter(Doc doc)
         {
             if (doc is ForceFlat)
@@ -87,22 +89,23 @@ void OnExit(Doc doc)
         docsStack.Push(document);
         while (docsStack.Count > 0)
         {
-            var doc = docsStack.Pop();
+            var doc = docsStack.Peek();
 
             if (doc == TraverseDocOnExitStackMarker)
             {
+                docsStack.Pop();
                 OnExit(docsStack.Pop());
                 continue;
             }
 
-            docsStack.Push(doc);
-            docsStack.Push(TraverseDocOnExitStackMarker);
-
             if (!OnEnter(doc))
             {
+                OnExit(docsStack.Pop());
                 continue;
             }
 
+            docsStack.Push(TraverseDocOnExitStackMarker);
+
             if (doc is Concat concat)
             {
                 // push onto stack in reverse order so they are processed in the original order