ES module support in loaded scripts

[mechite]

(transferred from ticket T-17073)

Uncaught SyntaxError: Failed to execute 'appendChild' on 'Node': Cannot use import statement outside a module

Can we make it possible that .js files which get added as <script> could be manually configured to be <script type="module"> ?
Use the well-established .mjs extension and automatically insert them as such, or register them in docs.json as modules.

This is available in all supported browsers (caniuse).
It will allow for the use of the import statement (and e.g. esm.run), as well as async/await directly.