Expose new_in_scope for signals

[ealmloff]

Exposes the new_in_scope function for signals which makes it possible for users to make a signal last longer than the current scope.

For example:

let signals = use_signal(cx, || vec![Signal::new(0)]);
// ...In a Child component...

// Because we use the origin scope of the array, even if the child scope is dropped, the signal will still exist as long as the array does
signals.push(Signal::new_in_scope(123, signals.origin_scope()));

[JaniM]

This is particularly important for me. Could we get this merged so I could stop using the branch and enjoy whatever master has? 😄