### Godot version

4.2 247c354

### System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 30.0.15.1403) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

### Issue description

#75778 exposed `_validate_property()` to scripting. It's implemented in GDScript and C#, but not GDExtension (depends on #78634).
https://github.com/godotengine/godot/blob/247c3548d810136ffe9c1694cd76db3236efaa90/core/object/script_language_extension.h#L685-L687

Relevant discussion: https://github.com/godotengine/godot/pull/75778#discussion_r1307294940

### Steps to reproduce

1. Add GDExtension script
2. Use `_validate_property()`

### Minimal reproduction project

N/A
const Variant *self = (const Variant *)p_self;
	memnew_placement(r_ret, String(*self));
}
static GDExtensionVariantType gdextension_variant_get_type(GDExtensionConstVariantPtr p_self) {
	const Variant *self = (const Variant *)p_self;
	return (GDExtensionVariantType)self->get_type();
}
static GDExtensionBool gdextension_variant_has_method(GDExtensionConstVariantPtr p_self, GDExtensionConstStringNamePtr p_method) {
	const Variant *self = (const Variant *)p_self;
	const StringName *method = (const StringName *)p_method;
	return self->has_method(*method);
}
static GDExtensionBool gdextension_variant_has_member(GDExtensionVariantType p_type, GDExtensionConstStringNamePtr p_member) {
	return Variant::has_member((Variant::Type)p_type, *((const StringName *)p_member));
}
static GDExtensionBool gdextension_variant_has_key(GDExtensionConstVariantPtr p_self, GDExtensionConstVariantPtr p_key, GDExtensionBool *r_valid) {
	const Variant *self = (const Variant *)p_self;
	const Variant *key = (const Variant *)p_key;
	bool valid;
	bool ret = self->has_key(*key, valid);
	*r_valid = valid;
	return ret;
}
static void gdextension_variant_get_type_name(GDExtensionVariantType p_type, GDExtensionUninitializedVariantPtr r_ret) {
	String name = Variant::get_type_name((Variant::Type)p_type);
	memnew_placement(r_ret, String(name));
}
static GDExtensionBool gdextension_variant_can_convert(GDExtensionVariantType p_from, GDExtensionVariantType p_to) {