Adds ability to use custom types for <fields> in SceneGraph

[markwpearce]

• Add basic lookups & parsing for custom types (interfaces)
• verify diagnostics related to how the type in the XML is parsed are visible
• Add/check Validation for incompatible usage
• What happens when a type of a field is defined in one scope but not in the other?
• Check on Unions, etc.
• Verify that transpiled XML is correct - do we need a method toSceneGraphTypeString() on BscType
• How do we handle when we're allowed to use Custom Types?