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

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