Rewrite transcript logic to be more generic

The transcript logic in Invidious was written specifically
as a workaround for captions, and not transcripts as a feature.

This commit genericises the logic a bit as so it can be used for
implementing transcripts within Invidious' API and UI as well.

The most notable change is the added parsing of section headings
when it was previously skipped over in favor of regular lines.

src/invidious/routes/api/v1/videos.cr

@@ -89,9 +89,14 @@ module Invidious::Routes::API::V1::Videos
 
     if CONFIG.use_innertube_for_captions
       params = Invidious::Videos::Transcript.generate_param(id, caption.language_code, caption.auto_generated)
-      initial_data = YoutubeAPI.get_transcript(params)
 
-      webvtt = Invidious::Videos::Transcript.convert_transcripts_to_vtt(initial_data, caption.language_code)
+      transcript = Invidious::Videos::Transcript.from_raw(
+        YoutubeAPI.get_transcript(params),
+        caption.language_code,
+        caption.auto_generated
+      )
+
+      webvtt = transcript.to_vtt
     else
       # Timedtext API handling
       url = URI.parse("#{caption.base_url}&tlang=#{tlang}").request_target