Use AST parsing for lint file checking (github#17471)

* Add markdown parsing dependencies

* Use it for the link test

* Use ast tests for hard-coded language

* Dedupe visiting for links

* Add new mdast utils to check-deps exclusions

package.json

@@ -65,6 +65,7 @@
     "linkinator": "^2.13.1",
     "liquid": "^5.1.0",
     "lodash": "^4.17.19",
+    "mdast-util-from-markdown": "^0.8.4",
     "mini-css-extract-plugin": "^0.9.0",
     "mkdirp": "^1.0.3",
     "morgan": "^1.9.1",
@@ -86,6 +87,7 @@
     "slash": "^3.0.0",
     "strip-html-comments": "^1.0.0",
     "style-loader": "^1.2.1",
+    "unist-util-visit": "^2.0.3",
     "uuid": "^8.3.0",
     "walk-sync": "^1.1.4",
     "webpack": "^4.44.0",