Add highlighting for Kotlin

### Is your feature request related to a problem? Please describe
I am one of the Java Advent organizers and with Kotlin's popularity going further through the roof it starts getting problematic that articles about Kotlin can't have properly highlighted code snippets.

### Describe the solution you'd like
In addition to `[java]`, a `[kotlin]` block (and similar) would be helpful

No files or tests are named. Start by locating the existing Java language support and how language blocks are registered, then add equivalent Kotlin support so a [kotlin] block highlights code; verify the plugin still handles [java] and the new Kotlin block correctly.

Written by the indexing model from the issue text.