Add support for M4U files and save playlists as M4U files

[ArtyIF]

Overview Description: The reason for me suggesting this change is that GNOME assumes M3U files are only meant for music playlists, and thus it always tries to open it with Rhythmbox. M4U is a valid file type that does the exact same thing, however, Celluloid tries to open it with MPV instead of treating it as a playlist, thus returning an "unrecognized file format" error.

Steps to Reproduce:

1. Open an M4U file in Celluloid
2. Get error

Actual Results: An error is shown:

Expected Results: Opening an M4U file would be equivalent to opening an M3U file.

Version: Celluloid 0.23

Additional Information: Installed via Flatpak on Fedora 36 and GNOME 42.2

[gnome-mpv]

This is something that should be fixed in mpv. That said, it doesn't look like M4U is a well-recognized format. I can't find any useful information on it at all.