GrammarLang: Add Mixins to Grammar language

[breck7]

Potentially a new node type: trait/interface to allow for multiple inheritance like abilities (css style).

[breck7]

not obviously a bad idea but more important things to do first.

[breck7]

Right now I'm often using inheritance for 2 unrelated things:

1. copying properties from a prototype/abstract class
2. bundling things into a scope (all inherit from a class, and then the parent scope class just needs to have 1 nodeType in scope)

I probably want to be able to create another way to put more node types in scope without the inheritance part. Some sort of tag.

[breck7]

Would be neat but medium pri