document how intellij bazel rules depend on scala rules

in this file there are two references to intellij:

https://github.com/bazelbuild/rules_scala/blob/master/scala/scala.bzl#L774

It seems that intellij seems to be wanting some information put in the struct accessible at `.scala` on a target. However, we want to move to modern providers, so that seems to mean that the intellij rules will need to be updated.

Can we document:

1. what dependency do the intellij rules have on us? What API exactly do they require?
2. what we need to do to change the API. I guess we will need to make changes in almost lockstep: so we make the breaking change here then send a PR to some other repo.

@ittaiz @gkk-stripe @andyscott do you all know anything about this?

Start with scala/scala.bzl at line 774 and inspect the two IntelliJ references. Document the exact API IntelliJ rules require from the .scala target struct, then describe the coordinated changes needed to move to modern providers and identify the other repository that would need an accompanying update.

Written by the indexing model from the issue text.