RELEASE: v12.15 (main), v12.10 (core, core-mt)

Merging a few more PRs.

• Export worker from ffmpeg package (thanks @ntnyq , #805 ), export worker in core-mt package as well (thanks @dumbmoron , #703 )
• Add solidstart / vite app (#765 , thanks @EmoPorEmilio)
• Fix SvelteKit example URL (#720, thanks @elvisvoer)
• Export types within @ffmpeg/ffmpeg (#723 , thanks @snelsi)