Add support for scala.js

[gvacaliuc]

Description

Adding the following rules:

• scalajs_library
• scalajs_binary
• scalajs_proto_library
  • with support for grpcweb

Example usage: https://github.com/gvacaliuc/bazel-scalapb-grpcweb-example

This currently works with unary rpc calls, but there is some mangled error that I believe to be from the from the minified grpc-web source on the client side. Confirmed that the server is streaming results back to the client, but the client does not process the returned stream.

I think that there is something called a "source map" which will let us debug from the minified JS to original source (which will help us debug the grpc-web error) and the minifed JS to the original SCALA (so you can debug your scala.js code). The scala.js linker has some options to configure it.

issues

• move scalajs rules to a seperate scala_js.bzl file
• figure out what a source map is and how we can plumb it through bazel to webpack
• determine if it makes sense to look into rules_nodejs for some of their provider values (so that scalajs_ rules accurately reflect what they provide rather than masquerading as JavaInfo providers
  • if so, add an example w/ esbuild to that above
• determine what the dev/prod lifecycle looks like (sbt has fastOpt/fullOpt)
• figure out how to write tests in scala.js
• figure out how to write tests for these rules
• clean up library so that we just override the compile phase
• the proto library methods conflict with each other because they introduce an aspect w/ the same name, namely they both codegen scala for their proto dependencies (can we share...?)
• fix issues w/ the scala dep provider toolchain (rules_scala build fails w/o a properly configured toolchain, e.g. one that is provided in the example above)

This is a good link to understand what's going on with scala.js: http://www.lihaoyi.com/hands-on-scala-js/#TheCompilationPipeline

Motivation

• https://github.com/ConsultingMD/potluck/pull/2653
• going to be freaking sweet when we build the lasik jupyterlab plugin

[gvacaliuc]

i think that there is a way to change the compile phase here w/o creating a whole new rule