Add prepend function for signals with Void element

[someyura]

Motivation

The existing prepend and append functions requires passing an instance of the Element type, which leads to the following code when the Element is Void:

var mySignal: Signal<Void, Never>()
...
mySignal.prepend(()) // we have to pass () to avoid compilation error
mySignal.append(()) // we have to pass () to avoid compilation error

that seems unnecessary as all instances of Void as the same.

Change

I've added a specialized extension with an overload for the prepend and append functions. It will allow calling them without passing any parameters.

var mySignal: Signal<Void, Never>()
...
mySignal.prepend() 
mySignal.append() 

Compatibility

This change is strictly additive and is fully compatible with existing code.

Tests

On a local run all tests were passed successfully ✅