handle stdpath separators on windows

[Kethku]

Note: this PR includes slightly different changes in the lua and fennel because I didn't know how to compile the fennel code on my windows machine without dipping into wsl, which I wasn't up for at the moment.

If you can give me some details about how to do the compilation for real, I will make them match up again.

[Olical]

There's a conflict with the develop branch and there was a small change I wanted to make so I'm just manually adding this change, will tag 😄

[Olical]

Closing since the change is merged through more manual means.