### 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
String *self = (String *)p_self;
	Char16String cs = self->utf16();
	GDExtensionInt len = cs.length();
	if (r_text) {
		const char16_t *s_text = cs.ptr();
		for (GDExtensionInt i = 0; i < MIN(len, p_max_write_length); i++) {
			r_text[i] = s_text[i];
		}
	}
	return len;
}
static GDExtensionInt gdextension_string_to_utf32_chars(GDExtensionConstStringPtr p_self, char32_t *r_text, GDExtensionInt p_max_write_length) {
	String *self = (String *)p_self;
	GDExtensionInt len = self->length();
	if (r_text) {
		const char32_t *s_text = self->ptr();
		for (GDExtensionInt i = 0; i < MIN(len, p_max_write_length); i++) {
			r_text[i] = s_text[i];
		}
	}
	return len;
}
static GDExtensionInt gdextension_string_to_wide_chars(GDExtensionConstStringPtr p_self, wchar_t *r_text, GDExtensionInt p_max_write_length) {
	if constexpr (sizeof(wchar_t) == 4) {
		return gdextension_string_to_utf32_chars(p_self, (char32_t *)r_text, p_max_write_length);
	} else {
		return gdextension_string_to_utf16_chars(p_self, (char16_t *)r_text, p_max_write_length);
	}
}
static char32_t *gdextension_string_operator_index(GDExtensionStringPtr p_self, GDExtensionInt p_index) {