Refactor directive block parsing for efficiency.

Replaced inline dictionary with static readonly `FrozenDictionary` and introduced an alternate lookup for better performance. Adjusted tests and trimmed parsing logic to align with the updated implementation.

Author: Mpdreamz

src/Elastic.Markdown/Myst/Directives/DirectiveBlockParser.cs

@@ -34,7 +34,7 @@ public DirectiveBlockParser()
 
 	private readonly string[] _codeBlocks = ["code", "code-block", "sourcecode"];
 
-	private readonly FrozenDictionary<string, int> _unsupportedBlocks = new Dictionary<string, int>
+	private static readonly FrozenDictionary<string, int> UnsupportedBlocks = new Dictionary<string, int>
 	{
 		{ "bibliography", 5 },
 		{ "blockquote", 6 },
@@ -63,17 +63,19 @@ public DirectiveBlockParser()
 		{ "seealso", 3 }
 	}.ToFrozenDictionary();
 
+	private static readonly FrozenDictionary<string, int>.AlternateLookup<ReadOnlySpan<char>> UnsupportedLookup =
+		UnsupportedBlocks.GetAlternateLookup<ReadOnlySpan<char>>();
+
 	protected override DirectiveBlock CreateFencedBlock(BlockProcessor processor)
 	{
 		var info = processor.Line.AsSpan();
 
 		if (processor.Context is not ParserContext context)
 			throw new Exception("Expected parser context to be of type ParserContext");
 
-		// TODO alternate lookup .NET 9
-		var directive = info.ToString().Trim(['{', '}', '`']);
-		if (_unsupportedBlocks.TryGetValue(directive, out var issueId))
-			return new UnsupportedDirectiveBlock(this, directive, issueId, context);
+		var directive = info.Trim(['{', '}', '`', ':']);
+		if (UnsupportedLookup.TryGetValue(directive, out var issueId))
+			return new UnsupportedDirectiveBlock(this, directive.ToString(), issueId, context);
 
 		if (info.IndexOf("{tab-set}") > 0)
 			return new TabSetBlock(this, context);

tests/Elastic.Markdown.Tests/Directives/AdmonitionUnsupportedTests.cs

@@ -11,11 +11,11 @@ namespace Elastic.Markdown.Tests.Directives;
 public abstract class AdmonitionUnsupportedTests(ITestOutputHelper output, string directive)
 	: DirectiveTest<UnsupportedDirectiveBlock>(output,
 $$"""
-  :::{{{directive}}}
-  This is an attention block
-  :::
-  A regular paragraph.
-  """
+:::{{{directive}}}
+This is an attention block
+:::
+A regular paragraph.
+"""
 	)
 {
 	[Fact]