Expand directory watcher globs containing ** ()

Expand directory watcher globs containing ** ()

Previously, if the server requested a glob pattern like foo/**/*
to be watched, we would just error.  Now we watch foo/bar/ and
foo/baz/ as if the server had requested those two watchers
instead of just the one with the **.

As a limitation, the implementation of file-expand-wildcards
doesn't fully handle ** globstars (** matches at most one path
segment).

* eglot.el (eglot-register-capability workspace/didChangeWatchedFiles):
Use file-expand-wildcards to make a ** glob into multiple **-less
globs.

GitHub-reference: https://github.com/joaotavora/eglot/issues/293