Support for Running in Workers without `document` Object

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.