gh-89727: add os.walkdir and os.fwalkdir

[jonburdo]

Add os.walkdir and os.fwalkdir, similar to os.walk and os.fwalk except they return os.DirEntry objects instead of string paths. They also handle symlinks to directories by putting them in nondirs rather than dirs if follow_links=False - like Path.walk rather than os.walk.

TODO:

• also return os.DirEntry for the first root directory (the top passed in)
• add follow_junctions=True kwarg
• support dir editing with generator.send

possible additional steps based on #89727 (comment) :

• move functions to os.path
• make on_error work more similarly to shutil.rmtree's onerror

• Issue: Change shutil.rmtree and os.walk to support very deep hierarchies #89727