Dispatch a FileSystem "change" event even when a dir change yields empty
added/removed lists - because there's a delay between the change occurring
and the time we re-read the dir contents in response, we might see
back-to-back changes (e.g. a delete-recreate pair) as a no-op even though
something definitely changed. See #6609 for an example bug.

Updated docs to clarify the two different special cases of directory change
events. (Also contains an unrelated small docs improvement in FileUtils).

ProjectManager already responds to this special case the way we'd want - it
calls FileSyncManager and then does nothing else - so no code changes
needed there.
Unit test for bug #6609 (Fast file replacement operations can be missed)

Enhance MockFileSystemImpl to allow artificially delaying watcher
notifications in addition to delaying normal callbacks.
Typo fix
Merge pull request #6625 from adobe/pflynn/dont-ignore-noop-dir-change

Copy of PR #6615 for release branch