[CLI] Add qmk list_keyboards (#6927)

`list_keyboards` replicates the `make list-keyboards` by globbing for all paths
that include `rules.mk` and then removing the paths that include `keymaps`.

This basis of this cli command could be reused in the future as a util, but is
not done so here since this would be the only place that would use it currently

Resolves #6911

lib/python/qmk/cli/__init__.py

@@ -8,6 +8,7 @@ from . import config
 from . import doctor
 from . import hello
 from . import json
+from . import list
 from . import new
 from . import pyformat
 from . import pytest