Reviving this because the only thing left here is to teach CSP to allow theme js uploads. js files have been default-allowed as theme uploads for a while now.
If theme js uploads are not blocked by CSP, then components like Image Annotator - Allows you to annotate images in the previewer won’t need to load their dependencies on the homepage (~170kb gzip). That component, for example, will only need to load those dependencies if the composer is opened. Plus, it never needs to load them to anon viewers.
Also, this change would “allow” themes to have web worker files that can do some heavy lifting off the main thread.
Allow in quotes above because you can have them as blobs, but it’s much nicer to have them in separate files instead of messing around with javascript in a string.