fix stringify not picking up negative zero if a normal zero has appeared before it#87

I found this while I was messing around with this package after I started doing some sveltekit prototyping. Just run the test I added in this PR without the included fix and notice how it stringifies it to just a normal zero. I think the reason this fails is because `indexes.has(thing)` will return true if `thing` is negative zero and `indexes` contains a value for normal zero. So my proposed fix is fairly simple: just first check for all the constants before checking the indexes map. Now I do not know if this has any unintended side-effects. So if I overlooked something feel free to deny this PR and cook up something else 😛. Now while I dont really need correct support for negative zero myself, since the readme explicitly mentions support for it, I figured might as well PR it 😄.