Refactor WebVTT building logic into WebVTT::Builder

[syeopite]

#4001 (comment)

Syntax is similar to JSON.build.

string = WebVTT.build do |vtt|
  vtt.cue(Time::Span.new(seconds: 1), Time::Span.new(seconds: 2), "Line 1")
  vtt.cue(Time::Span.new(seconds: 2), Time::Span.new(seconds: 3), "Line 2")
end

string # => "WEBVTT\n\n00:00:01.000 --> 00:00:02.000\nLine 1\n\n00:00:02.000 --> 00:00:03.000\nLine 2\n\n"

[syeopite]

The setting_fields here is mostly for backwards compatibility. I'm pretty sure — or at least I couldn't find any reference to it — that the fields Invidious adds aren't actually apart of WebVTT

[SamantazFox]

Yeah, it looks completely non-standard. I couldn't find anything about them on MDN

[SamantazFox]

You even wrote a spec for it <3

[SamantazFox]

Can you please rebase your changes?

[syeopite]

Rebased

[SamantazFox]

It works like a charm :D

[SamantazFox]

Thanks for your PR :D