This is a quick hack in response to !952 (closed).
Right now, to check if the X server is XWayland or not we rely on various properties. A more reliable way is to have an actual Xwayland-specific extension. Properties can be set by the client, initializing an extension can only be done by the server.
An extension doesn't need to provide events or errors (many existing don't) and it doesn't need to provide requests either. So we can just initialize an empty extension as a marker that this is XWayland.
The minor risk here is that if we every do want an XWayland extension and start adding requests, it'll become slightly more complicated for clients to figure out if it's the empty one or one that exists.