Add utility function mp.utils.process_path

Expected behavior of the wanted feature

For user scripts, sometimes it would be helpful knowing the process path, the script can know then if mpv or a frontend is used and what is the name and path of the frontend.

A use case for this is the thumbfast script, there are two frontends which have encoding support for thumbnail generation, that is ImPlay and mpv.net. The thumbfast script has an option to define the process path, unfortunately many users have difficulties to find and configure this option. If there would be a process_path utility function, the script could configure it automatically. This would also remove the necessity of mpv or the frontend being in the path env var, which is helpful for beginner users.

https://github.com/mpvnet-player/mpv.net/blob/main/docs/manual.md#user-data

https://github.com/po5/thumbfast?tab=readme-ov-file#mpv-frontends

https://github.com/po5/thumbfast/pull/113

Another method not used so far is using the LuaJIT FFI to get the process path.

@po5
@tsl0922

Start by locating the mp.utils entry point and review the linked mpv.net documentation and thumbfast usage for the expected process-path value. Done means user scripts can obtain the mpv or frontend executable path through mp.utils.process_path without requiring the executable to be present in PATH.