Move pathlib globbing implementation to a new module and class:
`pathlib._glob.Globber`. This class implements fast string-based globbing.
It's called by `pathlib.Path.glob()`, which then converts strings back to
path objects.

In the private pathlib ABCs, add a `pathlib._abc.Globber` subclass that
works with `PathBase` objects rather than strings, and calls user-defined
path methods like `PathBase.stat()` rather than `os.stat()`.

This sets the stage for two more improvements:

- pythonGH-115060: Query non-wildcard segments with `lstat()`
- pythonGH-116380: Move `pathlib._glob` to `glob` (unify implementations).