[Feature] Add afterCompile transform hook

[jtenner]

Use-case is dealing with reflection in my testing library. For example, given the following code snippet:

class Vec3 {
  constructor(
    public x: f64 = 0.0,
    public y: f64 = 0.0,
    public z: f64 = 0.0,
  ) {}
}

I would like to compile a few top-level statements:

// Just using this namespace as an example
Reflection.addClass(idof<Vec3>(), "Vec3");
Reflection.addClassProp(idof<Vec3>(), "x", offsetof<Vec3>("x"), Reflection.F64, 0 /* idof<T>() if reference*/);

I'm ready to do a lot of trial and error to reduce these down to something that looks like this:

Reflection.addClass(4, "Vec3"); // since id is known as a constant

Just need a way to push some statements to be parsed the backlog and this would be relatively easy.

module.exports = {
  afterCompile(parser: Parser): void  { // where do I get the class definition for this parameter?
    parser.managedClasses; // should be useful
  } 
}

Thoughts?

[MaxGraey]

Should be landed in #892