PS: Copy existing AST classes from internal repo

[MathiasVP]

This PR copies over all the existing AST classes from the internal repo. This is all done in the first commit. The remaining commits incrementally modernizes the AST classes and tests:

• efba031 changes the AST inheritance hierarchy to be more idiomatic by using extends instead of instanceof. Using instanceof will give a very bad UX when working with AST classes since the predicates defined in the base class won't be accessible when you have an instance of a subclass.
• 023c88a renames the long AST names to more standard CodeQL names. On the GitHub side experience has shown us that it's really really annoying to have to write:

  predicate foo(Expression e, Statement s) { ... }

  and we instead prefer being able to write:

  predicate foo(Expr e, Stmt s) { ... }

• 71349af adds more AST wrapper classes around the existing DB extensionals. I'm sure I'm still missing some, but these were some of the ones I spotted on a first pass 😂
• c30feab copies the existing AST-like tests from the internal repo.