By changing output esbuild setting for Markdown preview scripts to iife,
prevents global namespace pollution and reduces possibility of breaking
the preview by contributions from other extensions.
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
