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

---------

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"
-}
+}

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

@@ -0,0 +1,28 @@
+{
+    "fileTypes": [],
+    "injectionSelector": "L:markup.fenced_code.block.markdown",
+    "patterns": [
+        {
+            "include": "#math-code-block"
+        }
+    ],
+    "repository": {
+        "math-code-block": {
+            "begin": "(?<=[`~])math(\\s+[^`~]*)?$",
+            "end": "(^|\\G)(?=\\s*[`~]{3,}\\s*$)",
+            "patterns": [
+                {
+                    "begin": "(^|\\G)(\\s*)(.*)",
+                    "while": "(^|\\G)(?!\\s*([`~]{3,})\\s*$)",
+                    "contentName": "meta.embedded.math.markdown",
+                    "patterns": [
+                        {
+                            "include": "text.html.markdown.math#math"
+                        }
+                    ]
+                }
+            ]
+        }
+    },
+    "scopeName": "markdown.math.codeblock"
+}