Add scala_export

[vinnybod]

This adds a scala_export macro that works the same way as java_export and kt_jvm_export.
I'd like to add the scaladoc classifier artifact generation in a follow up PR.

[shs96c]

I'm wondering whether we should push both the kotlin_export and scala_export to their respective rulesets. We're introducing a circular dependency between those rulesets and rules_jvm_external, and we'd require people who are using these targets to have those rulesets available to them anyway.

Could you please open this PR against rules_scala, and I'll do the same for the kotlin rule.