fix(atoms): make Signal passable to functions accepting ZeduxNode

[bowheart]

Description

The Signal class's generics omit the Params and Template generics from the NodeGenerics accepted by the base ZeduxNode class. This is because Signals don't have Params and Template, but it means that a Signal can't be passed to a function accepting any ZeduxNode without some hacky TS workarounds.

To fix this, Signal needs to require the Params and Template generics. I fixed that. And since this is obnoxious for all consumer code, I've added an AnySignal type helper patterned after AnyAtomInstance &co:

// with the changes in this PR, anything accepting a Signal that only cares about the State would have to do this nonsense:
function acceptsSignal(
  signal: Signal<{
    Events: None;
    Params: undefined;
    State: MyRequiredStateType;
    Template: undefined;
  }>
) { ... }

// `AnySignal` to the rescue:
function acceptsSignal(
  signal: AnySignal<{ State: MyRequiredStateType }>
) { ... }

Add tests that verify existing types still work and these changes are working correctly. Also add tests ensuring all cross-assignments work - e.g. a MappedSignal can be passed anywhere a Signal or ZeduxNode is expected, same for AtomInstance, and a SelectorInstance can be passed anywhere a ZeduxNode is expected. In all cases, the passed node must match on any required generics.