Add Elm Test + CI

[nelsonic]

There is a glaring gap in this "starter": Elm Test!

Todo

• Add Elm Test to /assets/elm
  see: https://package.elm-lang.org/packages/elm-explorations/test/latest/
• Create a "hello world" test.
• Figure out how to run the Elm tests on GitHub Actions/Workflows CI.
• Document all the steps so that we can replicate this in a "real" project.

@SimonLab as discussed, if you have time, please add this. Thanks!

[nelsonic]

Potentially useful: https://github.com/mpizenberg/elm-tooling-action