Reapply parcel/watcher adoption

Revert "Revert e7fffbf1c9169087f1098aedfe54c59c079fa3ac"

This reverts commit 6786b0ad7dc638db71e4f219e9315ec5f2ae2298.

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