Store with start callback may double-call its first subscriber

[PixievoltNo1]

Steps to reproduce:

1. Create a writable/readable store with a start callback that synchronously calls its set callback. (Note: derived creates such a store when its own callback is synchronous.)
2. Subscribe with any number of subscribers, then unsubscribe them all.
3. Subscribe with a "new first subscriber".

Expected results: Each subscriber gets called exactly once upon subscribing, no matter what.

Actual results: The "new first subscriber" is called twice. REPL

Looking at the source code, I see set tries to guard against this, but it doesn't always work because stop isn't always falsy when needed. That could be fixed, but maybe we don't need the guard at all - why does subscribe add the subscriber before calling start? Can we safely reverse that?

[Rich-Harris]

Fixed in 3.6.0 — thanks