Migrate to Kotlin as a main development language

[madwareru]

Kotlin is a language which JetBrains initially designed to improve development of IDEs

Since a development of a plugin for IDE play in the same league, there is no reason to keep plugin code in plain old Java.

Kotlin is very easy to learn, and, among other things, it has nice features such as extension functions, which is tremendously improves interaction with PSI elements. It also has a very nice type system, which lets end user to do type inference, pattern matching and other things. All of it improves development experience hugely