multipleOf: support negative values

[ostrowr]

According to the spec, it's perfectly valid for a negative number to be a multipleOf a positive one:

https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-validation-00#rfc.section.6.2.1

[6.2.1.](https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-validation-00#rfc.section.6.2.1) multipleOf
The value of "multipleOf" MUST be a number, strictly greater than 0.

A numeric instance is valid only if division by this keyword's value results in an integer.

-4.5/1.5 = -3, which is an integer!

I found this because https://github.com/Stranger6667/jsonschema only admits positive multiples, which I don't think is correct per the spec, but has a 100% pass rate.

[karenetheridge]

nice one! LGTM.

[gregsdennis]

That looked right according to the spec.

We might consider reducing the positive requirement from multipleOf to just != 0 for the spec. I see no reason it needs to be positive.

[ostrowr]

That looked right according to the spec.

We might consider reducing the positive requirement from multipleOf to just != 0 for the spec. I see no reason it needs to be positive.

I think it's marginally simpler to leave it positive since it reduces the space of equivalent schemas. multipleOf: 1 and multipleOf: -1 are equivalent and so I think it's kind of nice to be opinionated about which one to use.