Bring back support for other language plugins

It seems that in the transition to the new Rust compiler we lost support for compiler language plugins. This is very sad. For a long time we have been using a language plugin to generate Scala.js code. Is there any way to bring this capability back?

No files, tests, or entry points are named. Start by locating the new Rust compiler integration and any prior language-plugin support, then determine how Scala.js generation was connected before the transition. Done means a documented, working path for other language plugins, including Scala.js.

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