Expose _validate_property() for scripting

[KoBeWi]

_validate_property() has (at least) 2 uses in scripting:

• convenient customization of exported property list (you can change usage without using _get_property_list()+_get()+_set())
• customizing built-in properties (hiding them, disabling etc. Right now it's not possible at all)

This PR exposes it to users. I did it only for GDScript, as I'm not sure how to do it in other languages.
I also added a call to _validate_property() for exported properties. This is now possible:

@export var test = 1

func _validate_property(property):
    if property.name == "test":
        property.usage |= PROPERTY_USAGE_READONLY

[YuriSizov]

I'm not sure how to do it in other languages.

I'm pretty sure this is why I didn't expose it in #64339. I may even have an old branch somewhere that does the same as you do, but the problem with passing property info to extensions remains.

[KoBeWi]

the problem with passing property info to extensions remains

Pretty sure it can be done the same way as in _get_property_list(). My problem was with how these functions are defined in extensions. It's a spaghetti of method pointer typedefs. I didn't even look at C#, because I could compile without it 🙃

[YuriSizov]

Pretty sure it can be done the same way as in _get_property_list().

For me the problem was that with validate it's a two way communication, whereas property list only returns a list of properties. For validate we need to construct GDExtensionPropertyInfo from PropertyInfo, pass it to extensions, and then somehow handle changes (is it a reference, is it a value?) Which was way beyond my comfort zone to implement. I'm sure it's doable :)

[KoBeWi]

I changed the calling order. Now the script _validate_property() is called after the virtual one, so users can validate already validated properties.

I also added a C# method stub.

[akien-mga]

Would be good to have this implemented before merging. CC @godotengine/gdextension

[dsnopek]

This'll require adding a new function pointer to GDExtensionScriptInstanceInfo, which (per the pattern being established in PR #78634) actually means making a new struct with the new property and a new version of the script_instance_create function in gdextension_interface.h. We've been holding off on these sort of changes until PR #78634 is merged, wanting to get at least the first one in before doubling down on this pattern.

In any case, I'd be happy to either write the code to do this (not sure the best way to provide it with this unmerged, though?) or to provide guidance on how to do it.

[akien-mga]

Makes sense. I guess let's wait for #78634, we could merge this PR with the TODO in the meantime, and make note / open an issue about implementing the GDExtension counterpart before the 4.2 release.

[akien-mga]

Looks good to me.

[akien-mga]

Thanks!