POC: Create a Trait-Based Model for the AST

[JLLeitschuh]

This AST allows API users to ask higher-level questions of the AST without requiring custom cursor traversal and visitors.

Questions like:

• What parameter index is this J.VariableDeclarations.NamedVariable?
• What callable is this Parameter a part of?

Future questions that could be asked are things like:

• Is this parameter effectively final?
• Where is this parameter read from?

The model for the traits matches, as closely as possible, CodeQL's