Highlighted code blocks inside the app (#1153)

app/Tests/Tools/CodeHighlighting/CodeMarkup.cs

@@ -0,0 +1,27 @@
+using System.Text;
+
+using AIStudio.Tools.CodeHighlighting;
+
+namespace AIStudio.Tests.Tools.CodeHighlighting;
+
+/// <summary>
+/// Writes code with its highlighted parts marked, such as {keyword:def}, so that a test reads like the
+/// code it checks.
+/// </summary>
+internal static class CodeMarkup
+{
+    public static string Of(string code, CodeLanguageRules rules)
+    {
+        var markup = new StringBuilder();
+        var position = 0;
+        foreach (var token in CodeTokenizer.Tokenize(code, rules))
+        {
+            markup.Append(code, position, token.Start - position);
+            markup.Append('{').Append(token.Kind.ToString().ToLowerInvariant()).Append(':').Append(code, token.Start, token.Length).Append('}');
+            position = token.Start + token.Length;
+        }
+
+        markup.Append(code, position, code.Length - position);
+        return markup.ToString();
+    }
+}