Add a --js-emit-wasm option and a corresponding using directive

[Quafadas]

• run ./mill -i generate-reference-doc.run in order to generate doc correctly
• check directive tags (should vs experimental)
• directive doc
• test
• pass CI

[Quafadas]

@Gedochao I think I might have gotten to the stage where I need some help / advice / review if I may once the CI has run through.

I added an emits WASM test. To actually run the WASM, I think we'd need node 22. Whilst I was rather gung-ho trampling the scala-ji-cli gha to add node 22, I'm far more hesitant about doing so for Scala CLI itself.

https://github.com/Quafadas/scala-cli/blob/cb64e000c0dbacc9e7ba831c83b85d82ba70c9b2/modules/integration/src/test/scala/scala/cli/integration/RunScalaJsTestDefinitions.scala#L327-L328

I'm proposing to not actually run the WASM emitted in this test suite, rather only test that the file exists. Execution is tested in the underlying js-cli project so it's arguably a duplication. If you feel strongly about it then I'll have to go through specifying some node versions...

The second thing, is that I noticed a test failure on windows BSP on the last time out. I don't think that is me, but I'm also not qualified to diagnose what's happening there. I think CI should go green for my set of changes. If it doesn't, I might need some help to figure out what's going on, if that's okay.

Finally, I'm not sure if any extra documentation is needed beyond what scala-cli demands as the minimal set it will pass CI with? Perhaps not, as sjrd covered that in the release notes of scala js itself?

Outside of those points, I believe this would be ready for review / merge ...

[Gedochao]

LGTM

[Gedochao]

@Quafadas thanks a lot for attending to this!

[Quafadas]

@Gedochao Pleasure - thanks for the support :-).