chore: Drop @excalidraw/extensions and move the MathJax subtype into

`src/excalidraw-app/subtypes` to leave `@excalidraw/excalidraw` untouched.

`@excalidraw/extensions` mostly contained boilerplate and obscured the
main new features here: `ExcalidrawElement` subtypes and MathJax support.

package.json

@@ -20,7 +20,6 @@
   },
   "dependencies": {
     "@braintree/sanitize-url": "6.0.2",
-    "@excalidraw/extensions": "link:src/packages/extensions",
     "@excalidraw/random-username": "1.0.0",
     "@radix-ui/react-popover": "1.0.3",
     "@radix-ui/react-tabs": "1.0.2",
@@ -41,6 +40,7 @@
     "image-blob-reduce": "3.0.1",
     "jotai": "1.13.1",
     "lodash.throttle": "4.1.1",
+    "mathjax-full": "3.2.2",
     "nanoid": "3.3.3",
     "open-color": "1.9.1",
     "pako": "1.0.11",
@@ -114,7 +114,7 @@
     "fix": "yarn fix:other && yarn fix:code",
     "locales-coverage": "node scripts/build-locales-coverage.js",
     "locales-coverage:description": "node scripts/locales-coverage-description.js",
-    "postinstall": "patch-package --patch-dir src/packages/extensions/patches",
+    "postinstall": "patch-package",
     "prepare": "husky install",
     "prettier": "prettier \"**/*.{css,scss,json,md,html,yml}\" --ignore-path=.eslintignore",
     "start": "vite",