GDExtension: Variant introspection methods missing

The engine has a lot of static helper methods on Variant, e.g:

```c++
static bool has_builtin_method(Variant::Type p_type, const StringName &p_method);

static ValidatedBuiltInMethod get_validated_builtin_method(Variant::Type p_type, const StringName &p_method);
static PTRBuiltInMethod get_ptr_builtin_method(Variant::Type p_type, const StringName &p_method);

static int get_builtin_method_argument_count(Variant::Type p_type, const StringName &p_method);
static Variant::Type get_builtin_method_argument_type(Variant::Type p_type, const StringName &p_method, int p_argument);
static String get_builtin_method_argument_name(Variant::Type p_type, const StringName &p_method, int p_argument);
static Vector get_builtin_method_default_arguments(Variant::Type p_type, const StringName &p_method);
static bool has_builtin_method_return_value(Variant::Type p_type, const StringName &p_method);
static Variant::Type get_builtin_method_return_type(Variant::Type p_type, const StringName &p_method);
static bool is_builtin_method_const(Variant::Type p_type, const StringName &p_method);
static bool is_builtin_method_static(Variant::Type p_type, const StringName &p_method);
static bool is_builtin_method_vararg(Variant::Type p_type, const StringName &p_method);
static void get_builtin_method_list(Variant::Type p_type, List *p_list);
static int get_builtin_method_count(Variant::Type p_type);
static uint32_t get_builtin_method_hash(Variant::Type p_type, const StringName &p_method);

```

These are very useful for the visual scripting plugin as they allow to introspect the objects and make decisions based on that. Could these be added to the GDExtension API?