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

Moving more functions into `path.c`. After this PR I will try to refactor `path.c` a bit and add more unit tests to it.

I could use some advice whether these functions belong in `os/fs.c` or `path.c`:
* `os_can_exe`
* `is_executable`
* `is_executable_in_path`