Add watch_filtered, filtering paths to watch

[daandemeyer]

Adds WatchFilter and Watcher::watch_filtered, allowing directories to be excluded when setting up a watch. Implemented for the inotify, fsevents, kqueue, windows, poll, and null backends, with filter support plumbed through Watcher::update_paths, notify-debouncer-full, and the FileIdCache API.

The filter gates directories only: walk-based backends prune excluded directories at scan time and gate directories discovered while watching; FSEvents and Windows suppress matching events at delivery time. File watches are never affected.

Two restrictions keep the semantics simple and the implementation small:

• Watching a directory the filter itself rejects returns the new ErrorKind::PathExcluded and changes nothing. Symlink roots are checked against their resolved target too.
• A directory watch carrying a filter must not overlap another directory watch in either direction; such calls are refused with an error. Filters are never merged across watches, so each watch entry carries exactly one filter. Accept-all watches keep the existing overlap semantics, file watches never conflict, and re-watching the same path still replaces the watch (WatchFilter::same_filter identity makes an identical re-watch a no-op).

Based on the filter-watch-targets branch:

• Add watch_filtered, filtering paths to watch.
• Add fsevent support and filter newly added paths too.
• Add support for watch_filtered to Windows.
• Add filtered watch implementation for kqueue.

[daandemeyer]

@JohnTitor Any chance you could enable workflows for this PR? The workflows don't run on my fork so I need them enabled here to make sure windows and macos work as expected.

[JohnTitor]

Triggered!

[daandemeyer]

@JohnTitor Hmm seems you need to reapprove every single time. I'll see if I can find some trivial fix that can already land so I don't have to ask every time

[daandemeyer]

Opened #951, if you merge that one CI will start working automatically here without needing your approval every time.

[daandemeyer]

Should be ready for review now, includes two other commits which I've split out into separate PRs. Will rebase this pr once those are merged. It's a lot of code, but a lot of it is tests, happy to split up into separate commits or PRs if that's preferred.

[daandemeyer]

Closing so I can open other prs