Generalize glob functionality.

[jaraco]

Inspired by work in python/cpython#106703.

Achieves:

• Adds support for newlines and fullmatch in the generated pattern.
• Adds support for arbitrary path separators, defaulting to platform separators.
• Fails when ** appears as anything but a full path segment.
• Requires at least one char for * in a full path segment.

[jaraco]

There's an issue with the Windows implementation, I worry it may not honor path separators the way glob does:

 draft @ mkdir cont
 draft @ touch cont/item.txt
 draft @ glob.glob('cont/*')
['cont\\item.txt']

(The regex produced will still only have / as the separator so wouldn't match cont\\item.txt)