Add a Delegator DSL compiler

[dduugg]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

Introduces a compiler for Delegtor subclasses. We currently only have two: LazyObject, with no custom methods, and Cask::URL which only delegates to attrs. Thus the compiler is fairly simple and does not even contemplate methods with parameters, all that is enough to replace the hand-rolled rbi file.

Resolves the TODO in Library/Homebrew/cask/url.rbi (cc @Bo98)

[MikeMcQuaid]

Thanks @dduugg!