Renaming Modes.DEFAULT_MODE to Modes.DEFAULT in the Kotlin runtime and target

[ftomassetti]

Currently we generate this code for the Kotlin target:

object Modes {
  const val DEFAULT_MODE = 0
  const val FOO = 1 // made uppercase by #8 
  const val BAR = 2
}

This is inconsistent because only the default mode has the _MODE suffix. We should therefore rename it, by working on the template for the Kotlin target and updating references in the Kotlin runtime

[ftomassetti]

I would assign the issue to me, but I have no permissions for doing so