Kotlin models should be able to utilize internal visibility

[anthonycr]

Summary

Currently, Kotlin support in stag requires public visibility of setters and getters. This is because internal, private, and protected all prevent stag from seeing the setters and getters. internal should be supported, but cannot until there is a standalone kotlin stag compiler due to the way setter and getter names are created in java code for kotlin.

Proposed Spec

• Kotlin fields should be able to reduce visibility of setters and getters to internal.