Add sorbet generation!

[dorner]

This:

• Adds a Tapioca compiler to generate RBI as a DSL. AND ALSO
• Adds an option, output_sorbet which will add Sorbet sig declarations to generated schema classes.
• Fixes some issues that were uncovered related to generation - e.g. required fields were output as optional, and test helpers wasn't printing out the correct diff.
• Removes RBS generation as I haven't seen any real support for it in the ecosystem.