gh-127001: Fix PATHEXT issues in shutil.which() on Windows (GH-127035)

* Name without a PATHEXT extension is only searched if the mode does not
  include X_OK.
* Support multi-component PATHEXT extensions (e.g. ".foo.bar").
* Support files without extensions in PATHEXT contains dot-only extension
  (".", "..", etc).
* Support PATHEXT extensions that end with a dot (e.g. ".foo.").