Add support for VTT (Web Video Text Tracks) file format

[vrcprl]

What

Implement a loader for VTT files in Replexica.

Why

To support web developers and video content creators working with HTML5 video subtitles.

How

1. Create a new loader in packages/cli/src/loaders/vtt.ts
2. Implement parsing logic for VTT structure
3. Add two unit tests in packages/cli/src/loaders/index.spec.ts:
   • Test parsing a simple VTT file
   • Test handling cue settings in VTT entries
4. Update @replexica/spec to include VTT format
5. Generate a changeset using pnpm new for both 'replexica' and '@replexica/cli' packages
6. In the pull request, include a link to the VTT specification used.

After completing the task

Noticed a file format Replexica doesn't yet support? Please create a new GitHub ticket (if it doesn't yet exist!), using the format/structure of this GitHub issue.

[partik03]

@maxprilutskiy I'll take this as vtt is very similar to srt file format