DEV: Fix wasm_bindgen double loading errors in Firefox (PR #14630)

When we are calling the loadLibs function, which in turn calls:

importScripts(settings.mozjpeg_script); importScripts(settings.resize_script);

For the media-optimization-worker service worker, we are getting an error in Firefox, which balks at wasm_bindgen, a global variable defined with let, being redefined when the module loads. This causes image processing to fail in Firefox when more than one image is uploaded at a time.

The solution to this is to just check whether the scripts are already imported, and if so do not import them again.

Chrome doesn’t seem to care about this variable redefinition and does not error, and it seems to be expected behaviour that the script can be loaded multiple times (see Handling duplicate importScripts() · Issue #1041 · w3c/ServiceWorker · GitHub)