fix: extract handlers and add cleanup function

[FredrikOseberg]

About the changes

This PR extracts handlers into their own functions and adds a returning function in the useEffect that cleans up the event listeners on unmount.

Closes #90
Closes #91

Important files

Discussion points

[thomasheartman]

Nice! This looks great to me 😄 Just one small question: It was pointed out in this comment on #90, that there might be a bug in checking the flag and having different handlers for ready and update:

I think there's actually a second (unrelated) small bug where the value of flagRef isn't being updated in the ready function. I think the impact would be that if a useFlag is used before the client is ready, and the flag is currently turned ON, then it won't respond if the flag later gets turned OFF (unless it's turned OFF then ON then OFF again).

It's then suggested to instead do this:

  useEffect(() => {
    if (!client) return;
    const updateFn = () => {
      const enabled = isEnabled(name);
      if (enabled !== flagRef.current) {
        flagRef.current = enabled;
        setFlag(enabled);
      }
    };
    client.on('update', updateFn);
    client.on('ready', updateFn);

    return () => {
      client.off('update', updateFn);
      client.off('ready', updateFn);
    };
  }, [client]);

and (from what I can tell) use the same function for both events. What do you think about that? Is that an issue?