Introduce cross-platform file-watching (#6855)

This adds cross-platform file-watching via the
[Notify](https://github.com/notify-rs/notify) crate. The previous
fs-events implementation is now only used on MacOS, and on other
platforms Notify is used. The watching function interface is the same.

Related to #5391 #5395 #5394.

Release Notes:

- N/A