Separate types for readonly Store (Store) and listen-only Event (Event)

[igorkamyshev]

Proposal

1. Mapped stores should not be modified, we can show it on type level:

const $s1 = createStore(''); // StoreWritable<string>
const $s2 = $s1.map(s => s.length); // Store<number>

sample({
  clock: someEvent,
  target: $s2,
})
// TYPE ERROR

Same for derived events

const derivedEvent = sample({ soure: $s, clock: event }); // Event<string>

sample({
  clock: someEvent,
  target: derivedEvent,
})
// TYPE ERROR

[sergeysova]

Store renamed into StoreWritable, but ReadonlyStore renamed into Store.

Event renamed into EventCallable, but ReadonlyEvent renamed into Event.

[YanLobat]

Resolved via #966