Simplify code in assets.js, remove mix of await and promise code

I would love to RTBC this, but don't feel remotely qualified. I don't know enough about how this works.

But, bumping to Major since this blocks #3228464: API for contrib projects to load CKEditor translations (which is Major and essential for the CKEditor 5 module to become stable.)

Looks good. Using async/await style everywhere in the file makes sense and makes it much easier to understand.

The longest part of reviewing this for commit was the time required to truly understand what assets.js was doing prior to the changes here. The fact that it took as long as it did for me to grok it helps justify the changes here. The changes here make the file significantly easier to understand. With these changes, this file may be maintainable by more than a select elite 🙂.

With these changes, this file may be maintainable by more than a select elite

+1!

#9++ — I don't see the risk in doing so? But maybe I'm missing something? 🤔

Wanted feedback on 9.3 and got it. This has been added to that version as well.

Yay! :)