Refactor IDL AST to use same AST as Interop Gen

[nikeokoronkwo]

Since the IDL generation and Dart Interop Interface Generation are both in the same package web_generator, this issue is to see if we can merge the ASTs used for generating Dart JS Interop code from IDL and TypeScript Declarations.

The IDL features will be worked in tandem with the Dart Interop Interface Gen features, and Dart Code Generation will be (temporarily) done under a separate function until we have completed all the features currently supported by the current IDL Generator.

This feature will also try to add support for any features currently not supported.

The following features are to be added:

Basic Support

• IDL Gen: Support Enumerations #395
• IDL Gen: Support Namespaces #398
• IDL Gen: Support Callbacks #401

Added Features

• IDL Gen: Support Function/Method Overloading #396

Refactoring/Fixes

• [web_generator] IDL Gen should use JSAny? instead of JSVoid for void types nested as a type parameter #397
• [web_generator] Generate all top-level declarations by default #400