Add additional enum methods

[derikson]

• Add negative scopes generated by enum.
• Add method that returns the mapping of enum name to database value.

Example:

class User < ApplicationRecord
  enum status: {
    temporary: 1,
    accepted: 2
  }
end

# RBS is generated for these scopes
User.not_temporary
User.not_accepted
# RBS is generated for the mapping method
User.status # => { temporary: 1, accepted: 2 }

[derikson]

Incidentally, is there a reason singleton method signatures are added to the model class instead of using extend GeneratedRelationMethods?