Refactor AttrConfig as descriptor extension

[Anaminus]

• Depends on #65.

Currently, there are several locations where a descriptor can be passed in, but an AttrConfig cannot. One option would be to combine RootDesc and AttrConfig under a common "config" type. However, AttrConfig only contains simple configuration data, which is very suitable for what extensions are designed for.

So, instead of maintaining a separate type, AttrConfig will be removed, and reimplemented as a descriptor extension:

{
    "$rbxmkFields": {
        "Attributes": {
            "Property": "AttributesSerialize"
        }
    }
}