[Full-stack Signals] Make signal operations thenable

[Legioth]

All client-side operations that may modify the value of a signal should return a thenable object that will be resolved once it has been confirmed that the operation has been processed on the server.

Note that this is a custom object with a then method rather than a full-blown Promise since the object will also have other properties. Most of the other properties are specific to the operation but there's also a universal cancel() function that will prevent the operation from being sent to the server if it hasn't already been sent (no guarantee that the cancellation will actually succeed). The thenable type can be void for now but will eventually be customized for various operations to reflect what the operation actually did.