IDL Gen: Support Callbacks

[nikeokoronkwo]

This issue is to add support in the (new) IDL generator to support callback declarations.

At the moment, callbacks are generated as typedefs equalling JSFunction.
We can improve this and make them similar to how they are to be implemented in the Dart JS Interop Interface generator by adding type information to them (i.e similar to TS Callable Interfaces).

callback VoidCallback = void ();
callback StringCallback = void (DOMString result);
callback Comparator = long (DOMString a, DOMString b);
callback Transformer = any (any input);
callback AsyncOperationCallback = undefined (DOMString status);

[srujzs]

This may be better supported when we have dart-lang/sdk#54557 instead.