feat: don't mutate windowSize object if sizes don't change

[FezVrasta]

Sorry, I wanted to open an issue rather than a PR because:

1. I don't know if you want to implement this
2. I don't have time to work on it with the chance you will reject it

But you have disabled issues for this repository, so here's a lame attempt.

Basically, I'd like to be able to just do:

const windowSize = useWindowSize();
useEffect(() => /* something on resize */, [windowSize]);