GH-126363: Speed up pattern parsing in `pathlib.Path.glob()` (#126364)

The implementation of `Path.glob()` does rather a hacky thing: it calls
`self.with_segments()` to convert the given pattern to a `Path` object, and
then peeks at the private `_raw_path` attribute to see if pathlib removed a
trailing slash from the pattern.

In this patch, we make `glob()` use a new `_parse_pattern()` classmethod
that splits the pattern into parts while preserving information about any
trailing slash. This skips the cost of creating a `Path` object, and avoids
some path anchor normalization, which makes `Path.glob()` slightly faster.
But mostly it's about making the code less naughty.

Co-authored-by: Tomas R. <tomas.roun8@gmail.com>