Add Scala support for Maven projects

[olafurpg]

Currently, the lsif-java index command can only index Scala projects using sbt. It would be nice to add support to index Scala projects using Maven as well.

There seem to be two establish Scala plugins for Maven

• scala-maven-plugin
• scalor-maven-plugin

I'm not sure which one is more widely used. Please comment if you have suggestions for which integrations to prioritize supporting.

[jeremyrsmith]

@olafurpg I think you've got the description for this one mixed up with Maven. Gradle has only one scala plugin.

[olafurpg]

@jeremyrsmith whoops, you're right! I fixed the issue title.