Allow implementing get_class_category in GDExtension

[Gallilus]

Change GetClassCategory so it is handled in native directly.

After #78392 I tried extending MyScriptExtension to include get_class_category while I was probably doing something wrong it seemed more consistent to also get_class_category true as a native function and let the extension handle it the way they want.

CC @dsnopek

[dsnopek]

I'm just not very familiar with how scripting languages are implemented in Godot, so I feel really under-qualified to comment on whether this is a good idea or not. I'm hoping to learn more about this soon (I have an experiment I want to try ;-)) but until then, I think we'll need feedback from someone who's more well versed this area.

However, as to the implementation, I do have one comment with regard to making this change in a backward compatible way (assuming we decide to do this)...

[dsnopek]

Discussed at the GDExtension meeting, and we think adding this makes sense! Still needs compatibility addressed per previous comment.

[dsnopek]

@Gallilus If this is something you're still interested in finishing, here's the steps that I think are needed to do it:

1. Rebase on latest master
2. We've already added a GDExtensionScriptInstanceInfo2 struct in master - you can just add your new get_class_category_func and free_class_category_func to that struct
3. And, there's already a gdextension_script_instance_create2 function too - you'll just need to add code to gdextension_script_instance_create() to set your new fields to nullptr when using the old struct

This is assuming that this will get merged for Godot 4.2 when we can still make changes to the 2 version of the struct -- it doesn't, then it will need to add a new 3 version of the struct and function.

Since we've already approved the general idea behind this PR at a previous GDExtension meeting, I think it should be a quick one to review and approve!

[Gallilus]

Tested it with the latest build.
Works when adding get_class_category to instance_info.
And there is no class category when not creating the function. For me, it used to crash. wanted to mention thist as some may expect the default implementation to take over.

[dsnopek]

Thanks, this is looking good!

Aside from my comment on the code, there's only one more little thing missing - in gdextension_script_create_instance() we need a line that does:

info_2->get_class_category_func = nullptr;

So that that value is set to nullptr when older GDExtensions use script_instance_create() with the old struct.

[dsnopek]

Thanks! This is looking good to me :-)

[YuriSizov]

Thanks!