fix: use platform path separator instead of /

[nik-rev]

I'm working on Git Blame for Helix and some people on Windows experienced issues where they don't see the blame (helix-editor/helix#13133)

I found the reason is because Gix uses / for path separator always, instead of using the platform's separator. This PR fixes that

note: the way I'm turning MAIN_SEPARATOR_STR into a byte is a bit awkward here, I wish there was some kind of MAIN_SEPARATOR_BYTE

[Byron]

Hi @nik-rev, thanks a lot for contributing!

I left a comment in the original helix PR explaining why this PR here has to be closed.
In short, BString relative paths are always slash-separated, and I think it's lacking its own type to make that clear.
Thanks for your understanding.

CC @cruessler