[WIP] Move stuff from `os/fs.c` to `path.c`

Moving more functions into `path.c`. After this PR I will try to make some refactorings and add more unit tests to `path.c`.

I need some advice if these functions belong in `os/fs.c` or `path.c`:
* `os_can_exe`
* `is_executable`
* `is_executable_in_path`