Allow substitutions in code blocks from global docset variables. (#685)

* Fix substitutions in code blocks

* Fix naming

Author: reakaleek

docs/syntax/substitutions.md

@@ -46,5 +46,9 @@ tar -xzf elasticsearch-{{version}}-linux-x86_64.tar.gz
 cd elasticsearch-{{version}}/
 ```
 
+```bash 
+echo "{{a-global-variable}}"
+```
+
 
 Here is a variable with dashes: {{a-key-with-dashes}}

src/Elastic.Markdown/Myst/CodeBlocks/EnhancedCodeBlockParser.cs

@@ -147,13 +147,21 @@ private static void ProcessCallOuts(StringLineGroup lines, string language, Enha
 
 			var span = line.Slice.AsSpan();
 
-			if (span.ReplaceSubstitutions(context.YamlFrontMatter?.Properties, out var replacement))
+			if (span.ReplaceSubstitutions(context.YamlFrontMatter?.Properties, out var frontMatterReplacement))
 			{
-				var s = new StringSlice(replacement);
+				var s = new StringSlice(frontMatterReplacement);
 				lines.Lines[index] = new StringLine(ref s);
 				span = lines.Lines[index].Slice.AsSpan();
 			}
 
+			if (span.ReplaceSubstitutions(context.Substitutions, out var globalReplacement))
+			{
+				var s = new StringSlice(globalReplacement);
+				lines.Lines[index] = new StringLine(ref s);
+				span = lines.Lines[index].Slice.AsSpan();
+			}
+
+
 			if (codeBlock.OpeningFencedCharCount > 3)
 				continue;