Allow configurable access modifier for top-level types from PInvokeGenerator

[saucecontrol]

Currently all generated types are public, but it would be nice to be able to generate internal types for inclusion within a single library that uses them.

[tannergooding]

This is probably simple enough to support with a config switch. It should likely support the basic *=access, SimpleName=access, and Fully.QualifiedName=access that other opptions have.