Fix VARIANT_ENUM_CAST to support enum class

[daniellopez2002]

Problem

VARIANT_ENUM_CAST assumes implicit conversion from enum values to int64_t when
encoding arguments. This is invalid for enum class types according to the C++
standard and fails to compile on MSVC.

Solution

Explicitly cast enum values to int64_t when encoding, matching the behavior
already implemented in VARIANT_BITFIELD_CAST.

Notes

• ABI-safe
• Backward-compatible
• Improves MSVC compatibility

Fixes #1917

[dsnopek]

Thanks!

We still don't "officially" support enum classes, although, if this is really all it takes to make them work in godot-cpp, the change is small enough that it could be alright. I remember when discussing binding enum classes in Godot itself, it turned out there were a lot more changes than initially expected

[daniellopez2002]

Can it be merged? Or does it need any changes or improvements?

[dsnopek]

It looks like Godot itself already basically has this cast via this code:

https://github.com/godotengine/godot/blob/98782b6c8c9cabe0fb7c80bc62640735ecb076d3/core/variant/method_ptrcall.h#L243-L246

We should eventually sync the changes from godotengine/godot#105231 into godot-cpp, but I think this is OK for now

[daniellopez2002]

Thanks a lot for the clarification and for the review!

This is actually my first time contributing a PR to an open source project, so I really appreciate the feedback and the approval. Happy to help later on if syncing with core changes becomes necessary.