### 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;
	Variant *iter = (Variant *)r_iter;
	bool valid;
	memnew_placement(r_ret, Variant(self->iter_next(*iter, valid)));
	*r_valid = valid;
}
static GDExtensionInt gdextension_variant_hash(GDExtensionConstVariantPtr p_self) {
	const Variant *self = (const Variant *)p_self;
	return self->hash();
}
static GDExtensionInt gdextension_variant_recursive_hash(GDExtensionConstVariantPtr p_self, GDExtensionInt p_recursion_count) {
	const Variant *self = (const Variant *)p_self;
	return self->recursive_hash(p_recursion_count);
}
static GDExtensionBool gdextension_variant_hash_compare(GDExtensionConstVariantPtr p_self, GDExtensionConstVariantPtr p_other) {
	const Variant *self = (const Variant *)p_self;
	const Variant *other = (const Variant *)p_other;
	return self->hash_compare(*other);
}
static GDExtensionBool gdextension_variant_booleanize(GDExtensionConstVariantPtr p_self) {
	const Variant *self = (const Variant *)p_self;
	return self->booleanize();
}
static void gdextension_variant_duplicate(GDExtensionConstVariantPtr p_self, GDExtensionUninitializedVariantPtr r_ret, GDExtensionBool p_deep) {
	const Variant *self = (const Variant *)p_self;
	memnew_placement(r_ret, Variant(self->duplicate(p_deep)));
}
static void gdextension_variant_stringify(GDExtensionConstVariantPtr p_self, GDExtensionUninitializedVariantPtr r_ret) {