Support for Running in Workers without `document` Object

[Yharooer]

At the moment, FFmpeg.wasm cannot be run on a Service Worker since Service Worker does not have access to document.

One reason this does not work is due to the usage of resolve-url which fails if the document object is not available.

One would also have to change the way the script is injected by using importScripts over document.createElement('script') if document is not available.

[polisen]

seconded - broken in webworkers, which is obviously a huge use case.

[Paul-Browne]

Bump... running ffmpeg off the main thread would be very useful. Hopefully this can be prioritized...