feat: add WebVTT converter

[bjesuiter]

Hey there, human here!

I wanted my openclaw to build itself a skill to download YouTube video transcripts, and it did so by downloading a vtt file from YouTube and adding a small python script to convert that to markdown.

I thought it might be a great addition to markit to support vtt to markdown files, with deduplication of rolling subtitles as used by YouTube.

So I vibe coded this PR, if you have any remarks or complaints, send them my way, I'll fix it! :)

Example

Input sample.vtt:

WEBVTT

00:00:00.000 --> 00:00:02.000
Hello world.

00:00:02.000 --> 00:00:04.000
This is a caption test.

Run:

markit sample.vtt -q

Output:

# Transcript

## Text

Hello world. This is a caption test.

## Timestamped Transcript

- [00:00:00.000] Hello world.
- [00:00:02.000] This is a caption test.

YouTube-style rolling captions are deduplicated, so cumulative cue fragments become one readable transcript instead of repeated text.

[bjesuiter]

Refinement pass pushed in 46971b9 (refactor: refine WebVTT parsing).

Summary:

• Made .vtt extension matching case-insensitive (.VTT now works).
• Made WebVTT MIME matching case-insensitive.
• Simplified skipped WebVTT block handling with a shared prefix list.
• Consolidated cue timestamp-tag stripping into one regex.
• Improved HTML entity decoding, including numeric and hex entities such as 🐟.
• Added test coverage for the new edge cases.

Verified locally:

• bun run check
• bun test (123 pass)
• bun run build