Adds support for Github-style fenced math blocks in markdown editor and preview (#213750)

* added support for markdown fenced math blocks

* original markdownEngine.ts

* Add fenced math block rendering to markdown-math

* Update dependencies

* custom parser for fenced math blocks

* custom parser for fenced math blocks

* reverted changes to extension.ts

* reverted all changes from prior implementations

* proper fence grammar implementation for math

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Co-authored-by: Matt Bierner <matb@microsoft.com>

extensions/markdown-math/syntaxes/md-math-block.tmLanguage.json

@@ -82,4 +82,4 @@
 		}
 	},
 	"scopeName": "markdown.math.block"
-}
+}