Support Rails7 style enum definitions

[tk0miya]

Since Rails7, the enum definition style has been changed. This uses a private a private method #_enum_methods_module to obtain the registered enum defintions to support both Rails5 and Rails7.

Thanks to @ksss. (refs: https://github.com/pocke/rbs_rails/issues/6)

This is yet another version of https://github.com/pocke/rbs_rails/pull/267 (static analytics approach).

• Pros
  • Very simple code
• Cons
  • Touch the internal of the Enum system

[tk0miya]

Now rebased on #274 .

[derikson]

Unfortunately, this approach doesn't work when instance_methods: false in the enum options. However, I think it's preferable to parsing both the old definition style and the new definition style, and it handles enums defined outside the model file.