warn when heading contains inline applies_to

Author: colleenmcginnis

src/Elastic.Markdown/Myst/InlineParsers/HeadingBlockWithSlugParser.cs

@@ -29,16 +29,20 @@ public void Setup(MarkdownPipelineBuilder pipeline) =>
 	public void Setup(MarkdownPipeline pipeline, IMarkdownRenderer renderer) { }
 }
 
-public class HeadingBlockWithSlugParser : HeadingBlockParser
+public partial class HeadingBlockWithSlugParser : HeadingBlockParser
 {
 	private static readonly Regex IconSyntax = IconParser.IconRegex();
+	private static readonly Regex AppliesToSyntax = HeadingAppliesToParser.AppliesToSyntaxRegex();
 
 	public override bool Close(BlockProcessor processor, Block block)
 	{
 		if (block is not HeadingBlock headingBlock)
 			return base.Close(processor, block);
 
 		var text = headingBlock.Lines.Lines[0].Slice.AsSpan();
+		if (AppliesToSyntax.IsMatch(text))
+			processor.GetContext().Build.Collector.EmitWarning(processor.GetContext().MarkdownSourcePath, "Do not use inline 'applies_to' annotations with headings. Use a section 'applies_to' annotation instead.");
+
 		// Remove icon syntax from the heading text
 		var cleanText = IconSyntax.Replace(text.ToString(), "").Trim();
 		headingBlock.SetData("header", cleanText);
@@ -79,3 +83,9 @@ public static partial class HeadingAnchorParser
 	[GeneratedRegex(@"\$\$\$[^\$]+\$\$\$", RegexOptions.IgnoreCase, "en-US")]
 	public static partial Regex InlineAnchors();
 }
+
+public static partial class HeadingAppliesToParser
+{
+	[GeneratedRegex(@"\{applies_to\}`[^`]*`", RegexOptions.Compiled)]
+	public static partial Regex AppliesToSyntaxRegex();
+}