Add watcher filter

[kevinyang372]

Base PR: notify-rs#632

This adds implementation for:

• FSEventWatcher (Mac)
• INotifyWatcher (Linux)
• ReadDirectoryChangesWatcher (Windows) -- this was not included in the original PR. I had to implement it myself 😅 Verified it does compile on Windows

It doesn't include filter implementation for PollWatcher or KqueueWatcher since these are not used for any major OS. But they should still compile