feat: webvtt subtitles

[includeamin]

What and why

How it was tested

Checklist

• make ci passes
• There is a test that fails without this change
• Anything read from a file or a mapper is bounded before it is allocated
• The handbook is updated if operators, mapper authors, or players see the change
• Every commit is signed off (git commit -s)
• I did not copy or adapt code from a project with an incompatible licence (see CONTRIBUTING.md)