Rework Variant.h

[CedNaru]

I noticed that we never added the new PackedVector4Array to the Variant.h.
So I just took the opportunity to rework the file a bit.

The function bodies are almost unchanged, only the general structure of the file changed. Instead of the KtVariant namespace, I have put all methods into the new BufferToVariant and VariantToBuffer structures. Because everything is static, the structures act like a more fine-grained namespace. This is also how Godot does it through its codebase.

Note that I also removed the initializer function for the array of function pointers. It's now done statically, no "if" is required during runtime anymore.

Before :

    static void init_to_kt_methods(void (*to_kt_array[Variant::Type::VARIANT_MAX])(SharedBuffer*, const Variant&)) {
      // Long initializer...
     }

    static void send_variant_to_buffer(const Variant& variant, SharedBuffer* byte_buffer) {
        // must match the value order of godot_variant_type
        static void (*TO_KT_VARIANT_FROM[Variant::Type::VARIANT_MAX])(SharedBuffer*, const Variant&);
        if (unlikely(!TO_KT_VARIANT_FROM[0])) { init_to_kt_methods(TO_KT_VARIANT_FROM); }
        Variant::Type type = variant.get_type();
        TO_KT_VARIANT_FROM[type](byte_buffer, variant);
    }

After:

    static void write_variant(const Variant& variant, SharedBuffer* byte_buffer) {
        // must match the value order of godot_variant_type
        static void (*variant_writers[Variant::VARIANT_MAX])(SharedBuffer*, const Variant&) =  {
              // Long static initializer...
        };

        Variant::Type type = variant.get_type();
        variant_writers[type](byte_buffer, variant);
    }