Add Enum DSL Compiler

[coreyja]

Closes #3

As discussed in that issue we at Wellsheet are in the proccess of migrating from sorbet-rails to tapioca dsl. And one issue for us was our heavy use of ActiveRecord enums and the T::Enum classes that SorbetRails was generating passed on those

This PR adds both a new DSL compiler ActiveRecordTypedEnum and a new Railtie EnumsRailtie.

One thing this PR does NOT yet include, but was mentioned on the issue is a way to make this behavior opt-in. It currently follows the pattern of RelationsRailtie which is included by default.
I want to think about the best way to make this opt-in, and thought I'd get this draft up and see if anyone else has ideas as well!

[coreyja]

We purposefully use the defined_enums method from ActiveRecord, as opposed to using the definitions hash that is passed in

This way we can rely on the normalization that happens when the enums are defined. For instance enum status: [:created, :in_progress] is a valid way to define an enum. And the defined_enums normalizes that so we get a hash like { 'status' => { 'created' => 0, 'in_progress' => 1 } }
Allows us to always expect string keys, and know that we will have the correct values for each variant, even in Rails defined them for the user!

[stathis-alexander]

Hey @coreyja ,

Thanks a bunch for this contribution. I think there must be a way to set this up to configure which railties get applied, just need to figure out how to do it in an elegant way. I will need to experiment to figure out what that is (not sure if you have an idea here, but I was thinking of something similar to how Datadog instrumentation is configured).

I'm pretty swamped with other work at the moment, but will try to find some time to jam on this in the next week or two