Use `Path` instead of `str` and validate user provided paths

[kunaltyagi]

I'd prefer these to be valid Path to remove a bunch of corner cases due to using str (eg: some filesystems don't allow unicode names, but others do), but let's postpone that to later

Originally posted by @kunaltyagi in https://github.com/kunaltyagi/nsiqcppstyle/pull/35#discussion_r913044380