Make foo.$ an alias for foo() in the returned Accessors for createSignal() and createMemo()

[samualtnorman]

As in for the accessors returned by createSignal() and createMemo(), Object.defineProperty() a getter that simply calls the function.

For example you could have a "dollar-ize" function:

const $ize = accessor => Object.defineProperty(accessor, `$`, { get: accessor })

That you simply wrap around accessors:

-  return [readSignal.bind(s), setter];
+  return [$ize(readSignal.bind(s)), setter];

This will be a read-only getter, no setter to maintain read/write segregation.

This fixes type narrowing in TypeScript where you are otherwise forced to use the error-prone postfix bang (!) operator:

 const [ foo, setFoo ] = createSignal<Foo | undefined>()
 
 // ...

 return <>
-	{foo() && <p>{foo()!.bar}</p>}
+	{foo.$ && <p>{foo.$.bar}</p>}
 </>

Maybe this could be expanded to other accessors from other functions in the future, but for now I think putting it on just createSignal() and createMemo() is a minimal change that has a big impact.

There is already precedent for getter-based reactivity in the form of props.foo and resource.latest.

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I've seen a similar discussion but this looks like it requires compilation and to make the Accessor a little magical. That's not what I'm asking for, my suggestion doesn't touch compilation.