Support `...` terminating Yaml FrontMatter (#23195)

* FrontMatter rendered properly on markdownEngine

* FrontMatter terminates for 3 - or .

* Regex applied on both files for markdown

extensions/markdown/src/markdownEngine.ts

@@ -22,7 +22,7 @@ interface MarkdownIt {
 	utils: any;
 }
 
-const FrontMatterRegex = /^---\s*[^]*?---\s*/;
+const FrontMatterRegex = /^---\s*[^]*?(-{3}|\.{3})\s*/;
 
 export class MarkdownEngine {
 	private md: MarkdownIt;

extensions/markdown/syntaxes/markdown.tmLanguage

@@ -3551,7 +3551,7 @@
 			<key>begin</key>
 			<string>\A-{3}\s*$</string>
 			<key>while</key>
-			<string>^(?!-{3}\s*$)</string>
+			<string>^(?!(-{3}|\.{3})\s*$)</string>
 			<key>patterns</key>
 			<array>
 				<dict>

extensions/markdown/syntaxes/markdown.tmLanguage.base

@@ -1184,7 +1184,7 @@
 			<key>begin</key>
 			<string>\A-{3}\s*$</string>
 			<key>while</key>
-			<string>^(?!-{3}\s*$)</string>
+			<string>^(?!(-{3}|\.{3})\s*$)</string>
 			<key>patterns</key>
 			<array>
 				<dict>