Add support for ActiveRecord enums

[Envek]

Output enum values in TS types.

Given that ActiveRecord model declares enum column:

class Post < ApplicationRecord
  enum category: [:news, :article, :blog].index_by(&:itself)
end

Output enum keys as a possible values instead of generic type.

-  category?: string | null;
+  category?: "news" | "article" | "blog" | null;

typelize directive accepts enum option that overrides enum values from the model or suppresses it if false is given.

typelize category: [:string, enum: %i[whatever else]]