Add support for native PGS subtitle rendering without transcoding

[Arcus92]

Changes This PR allows the client to request and render native PGS subtitle streams without transcoding. PGS is a graphical subtitle format. The subtitles are rendered locally by the client on-top of the video element.

This adds a new dependency libpgs-js. I just created this library for the Jellyfin project. If desired, you can fork it into the @jellyfin domain.

Tasks

• [x] Disable native PGS support if 'Subtitle burn-in' is not set to 'auto'.
• [x] Improve performance by using WebWorkers to render subtitles in a separate thread.

Related Pull Request (backend): https://github.com/jellyfin/jellyfin/pull/12056 Related Pull Request (vue): https://github.com/jellyfin/jellyfin-vue/pull/2404