feat: Tolerate string literals w/ ppath.join / ppath.resolve

[arcanis]

What's the problem this PR addresses?

It's currently a little annoying to work with the fslib in tests, because all literal paths have to explicitly convert paths into PortablePath or Filename when passing them to ppath.join or ppath.resolve - even if they are literal strings, which are highly unlikely to be non-portable.

How did you fix it?

New generic overrides for ppath.join and ppath.resolve now tolerate string literals as arguments.

This is only true for those two methods; I didn't find a reasonable way to achieve it for all the FakeFS methods without all implementations having to copy / paste the signature.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[merceyz]

Seems some more instances were missed, searching for ppath.(join|resolve).*as (Filename|PortablePath) shows a few that could be updated.