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.

tsconfig-types.json

@@ -1,5 +1,5 @@
 {
-  "include": ["src/packages/excalidraw", "src/packages/extensions", "src/global.d.ts", "src/css.d.ts"],
+  "include": ["src/packages/excalidraw", "src/global.d.ts", "src/css.d.ts"],
   "compilerOptions": {
     "types": ["vite/client", "vite-plugin-svgr/client"],
     "allowJs": true,