Add support for Kotlin files

[psybers]

This merges the Kotlin branch to add support for Kotlin source files in the back-end data generation as well as the default visitors and a kotlin langmode in the query language.

[psybers]

Looks like there are 2 things that need finished before this merges:

1. the prettyprint() for Kotlin is not done
2. the backend visitor sets all files to KLS15 - we need to detect versions based on features used and set accordingly