fix service worker portable classes

[s-cork]

problem before - the decorator wasn't returning the cls 🤦

But I think this should mostly work i.e. making something portable will actually register it with kompot

But I think all portable classes will need to be registered with kompot from the server perspective So yeah

doing kompot.register(cls) will likely be necessary anyway

[s-cork]

Adjusted the PR to just make anvil.server.portable_class actually return the class

This means that things should be explicitly registered with kompot @kompot.register should work fine as an additional decorator on a portable classes