Extend a signal API to match the Svelte store contract

[kolodziejczak-sz]

First of all, thank you for the library!

I'd like to suggest a proposal to extend the usage of the signals-core library to the Svelte framework.
I mean an implementation of a Svelte's store contract, which is about a subscribe method: 

type Subscriber<T> = (value: T) => void;
type Unsubscriber = () => void;
subscribe(run: Subscriber): Unsubscriber;

So, the signals would need to have an additional subscribe method that returns a function that lets you unsubscribe.
The Nanostore library does exactly that: https://github.com/nanostores/nanostores#svelte, so developers can use atoms without any dedicated adapter.
I'm sure the advantages of the ability to subscribe to a certain signal would exceed the Svelte support.

More info about Svelte's stores can be found here:
https://svelte.dev/docs#run-time-svelte-store
https://github.com/sveltejs/svelte/blob/master/src/runtime/store/index.ts

Thanks

[marvinhagemeister]

@kolodziejczak-sz this is a great idea! Went ahead and implemented that in #134 . Signals can be used natively with Svelte with the latest version 🎉