Reapply parcel/watcher adoption

Revert "Revert e7fffbf1c9169087f1098aedfe54c59c079fa3ac"

This reverts commit 6786b0ad7d.

Two changes:

- Lazy import parcel/watcher
- Add `@parcel/watcher` as a dev dep in extensions folder so that we pull in correct version for build os

extensions/package.json

@@ -10,6 +10,7 @@
     "postinstall": "node ./postinstall.mjs"
   },
   "devDependencies": {
+    "@parcel/watcher": "2.0.5",
     "esbuild": "^0.11.12",
     "vscode-grammar-updater": "^1.0.4"
   }