Add new validator notValue

[lo1tuma]

Fixes #193

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[fabian-hiller]

Thank you! I will review your code as soon as I find some time.

[fabian-hiller]

What do you prefer? Is there an argument for one or the other?

// Option 1
const Schema1 = string([notValue('foo')]);
const Schema2 = string([notLength(10)]);

// Option 2
const Schema3 = string([valueNot('foo')]);
const Schema4 = string([lengthNot(10)]);

[lo1tuma]

I would prefer option 1, although I don’t have very strong opinion on that.

[fabian-hiller]

Thanks. At the moment, I think that's better, too. I'll try to review this PR over the weekend.