GH-127456: pathlib ABCs: add protocol for path parser (#127494)

GH-127456: pathlib ABCs: add protocol for path parser (#127494)

Change the default value of `PurePathBase.parser` from `ParserBase()` to
`posixpath`. As a result, user subclasses of `PurePathBase` and `PathBase`
use POSIX path syntax by default, which is very often desirable.

Move `pathlib._abc.ParserBase` to `pathlib._types.Parser`, and convert it
to a runtime-checkable protocol.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>