fix(#439): add ability to merge namespace enum as static nested enum class

We didn't have logic related to merging nested inside namespace with class if the namespace contains an enum. This PR adds this logic to move namespace enum inside Kotlin class as a nested static enum class.

Example:

// index.d.ts
export class Foo {}
export namespace Foo {
    enum Bar {
         A,
         B
     }
}

// Current result
external open class Foo

// After those changes
external open class Foo {
    enum class Bar {
        A,
        B
    }
}

Kotlin / dukat

fix(#439): add ability to merge namespace enum as static nested enum class #449