always split dib cells on markdown language

Author: brettfo

src/Microsoft.DotNet.Interactive.Documents.Tests/CodeSubmissionFormatTests.cs

@@ -11,7 +11,7 @@ public class CodeSubmissionFormatTests : DocumentFormatTestsBase
     {
         public InteractiveDocument ParseDib(string content)
         {
-            return CodeSubmission.Parse( content,"csharp", KernelLanguageAliases);
+            return CodeSubmission.Parse(content, "csharp", KernelLanguageAliases);
         }
 
         public string SerializeDib(InteractiveDocument interactive, string newLine)
@@ -193,11 +193,13 @@ public void empty_lines_are_removed_between_cells()
                 });
         }
 
-        [Fact]
-        public void markdown_cells_can_be_parsed_even_though_its_not_a_kernel_language()
+        [Theory]
+        [InlineData("markdown")]
+        [InlineData("md")]
+        public void markdown_cells_can_be_parsed_even_though_its_not_a_kernel_language(string cellLanguage)
         {
-            var notebook = ParseDib(@"
-#!markdown
+            var notebook = ParseDib($@"
+#!{cellLanguage}
 
 This is `markdown`.
 ");

src/Microsoft.DotNet.Interactive.Documents.Tests/DocumentFormatTestsBase.cs

@@ -17,7 +17,6 @@ protected DocumentFormatTestsBase()
                 new("csharp", new[] { "cs", "C#", "c#" }),
                 new("fsharp", new[] { "fs", "F#", "f#" }),
                 new("pwsh", new[] { "powershell" }),
-                new("markdown", new[] { "md" }),
             };
         }
     }

src/Microsoft.DotNet.Interactive.Documents/CodeSubmission.cs

@@ -57,7 +57,9 @@ void AddElement()
                 }
             }
 
-            var mapOfKernelNamesByAlias = kernelNames.ToMapOfKernelNamesByAlias();
+            var splittingKernelNames = kernelNames.ToList();
+            splittingKernelNames.Add(new KernelName("markdown", new[] { "md" })); // not a kernel language, but still a valid cell splitter
+            var mapOfKernelNamesByAlias = splittingKernelNames.ToMapOfKernelNamesByAlias();
 
             foreach (var line in lines)
             {