Implement Golem Scala plugin and exporter macro

[danieletorelli]

This implements an SBT plugin that comes with a macro and with ScalaJS dependencies and settings preconfigured to avoid any ScalaJS boilerplate to the user.

The user can just add the @Worker annotation to the worker object and then run sbt fullLinkJS to have it ready.

Please check the README file and the SBT tests to see how it works more in details.

The macro is simple, and can be surely improved, but I think is a good starting point.

[vigoo]

How is this project integrating with https://github.com/golemcloud/golem-scalajs-wit-bindgen ?

[danieletorelli]

How is this project integrating with https://github.com/golemcloud/golem-scalajs-wit-bindgen ?

@vigoo Indeed my idea would be to extend the work done in this PR to eventually be able to produce WASM directly.

The steps would be to:

• Produce JS code via ScalaJS (this is done here)
• Call https://github.com/golemcloud/golem-scalajs-wit-bindgen binary to produce a WIT file, like https://github.com/golemcloud/golem-examples/blob/main/examples/scala2/scala2-shopping-cart/build.sbt#L17-L21 does
• Call ComponentizeJS to generate WASM from the JS file of the first step and the WIT file of the second step, more or less like https://github.com/golemcloud/golem-examples/blob/main/examples/scala2/scala2-shopping-cart/build.sbt#L25-L30

I still have to think about it in details, but I've done all of this manually and it works. Now it would be just matter of automating the process as I described and then make it enough configurable / extensible to cover more complex use cases.