should `plusOrMinus` allow zeros and negative valu...

# kotest-contributors

[Alex Kuznetsov]

should plusOrMinus allow zeros and negative values? currently the following works, but should it:

nowAtUtc shouldBe (nowAtSwiss plusOrMinus 0.milliseconds)

[Alfonso Ristorato]

I guess the 0 makes sense if you want to test complete equality, the negative numbers are probably redundant tho

[LeoColman]

They are `abs`ed anyways, aren't they? We could throw to alert users they're being redundant but I don't find it really necessary