mux: implement webvtt

A generic raw WebVTT output muxer.

The original module is from @fcartegnie. I took the liberty to format the code style and split some functions to help with the raw text support. The module was also adapted to the addition of vlc_tick_t and vlc_frame_t to libvlccore.

For some context, this module is needed to support WebVTT subtitles in the HLS stream output which should come in an MR next week!