Allow extra js modules to be included in frontend

[thomasloven]

Description:

HTML imports are being deprecated, and the recommended way forward is to use js module imports instead. This allows specifying modules that will be imported in the frontend.

A use case may be adding aditional icon sets. This could be done using lovelace modules, but then the new icons would work only after loading the lovelace panel. I.e. in a browser opened directly to e.g./states or /config/entity_registry the new icons wouldn't show up.

Corresponding frontend PR: home-assistant/frontend#3288

(Quick and dirty WIP/POC)

Related issue (if applicable): N/A

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#9674

Example entry for configuration.yaml (if applicable):

frontend:
  extra_module_url:
    - /local/mything.js
  extra_module_url_es5:
    - /local/mything_for_older_browsers.js

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[ghost]

Hey there @home-assistant/frontend, mind taking a look at this pull request as its been labeled with a integration (frontend) you are listed as a codeowner for? Thanks!

This is a automatic comment generated by codeowners-mention to help ensure issues and pull requests are seen by the right people.

[balloob]

Please add a PR for the docs 👍

[thomasloven]

Will do!

[thomasloven]

home-assistant/home-assistant.io#9674
Please help me check the nomenclature. It's midsummer eve in Sweden, so I'm not exactly sober...