metro-file-map: Recrawl a directory after an unnamed change in FallbackWatcher

[robhogan]

Summary:
On Windows, libuv reports a change with no filename when changes to a watched directory overflow the 4KB buffer they're read into (fs-event.c). That's easy to hit when a package manager writes a new package into node_modules, and when it happens we have no idea which entries changed, or how many.

FallbackWatcher handled these with sane's #detectChangedFile heuristic: lstat every file registered in the directory and report either the first one that's missing or the most recently modified one. So at most one change was reported, any others were missed until restart, and for a directory with no registered files yet (like a newly created package directory) the event was dropped entirely.

This diff replaces that heuristic. On an unnamed change, FallbackWatcher lists the directory to watch and register anything new, so later changes under it are reported, then emits a recrawl event for it. FileMap already handles recrawl (NativeWatcher emits it for directory renames) by crawling that subtree and applying the difference, so additions, modifications and deletions are all picked up. recrawl goes through the usual debounce, so a burst of overflows costs one crawl.

Follows expo/expo#49363, which lists a new directory on an unnamed change in Expo's fork of metro-file-map. That covers the new-directory case only.

Changelog:

 - **[Fix]**: Fix the fallback (Linux/Windows) watcher missing changes when Windows overflows its change buffer for a directory

Test plan:
Two new tests in FallbackWatcher-test.js call the fs.watch listener for a directory with no filename, as libuv does on overflow, with that directory's (and the root's) real events withheld. On main neither the recrawl nor the watch on a new subdirectory happens; both pass with this diff. Run on macOS only - I haven't reproduced a real overflow on Windows.

yarn jest packages/metro-file-map
yarn flow check
yarn eslint packages/metro-file-map/src/watchers

[robhogan]

Folded into #1967.