[9.x] Add whereNot method to Fluent JSON testing matchers

[mateusjatenee]

I have a use case where I want to test that a given item does not exist on a JSON collection.
I found it a bit difficult to do this with the existing API, but it is quite possible (and likely) that I just missed something.

Here's an example of my use case: https://twitter.com/mateusjatenee/status/1550574619995815936

The tests are a bit messy because I started out scoping then and then implementing (tried out a couple different things as well), but if the PR looks good I can make them a little better.

[mateusjatenee]

Whoopsie. Will check tests when I get back home.

[morloderex]

@mateusjatenee How is this different than the missing method with a potentially new callback parameter added to it?

[mateusjatenee]

@morloderex if we could figure out a nice way to handle the callback I don't think it'd be much different. The goal is to just make sure that a certain item does not exist (by filtering) — right now missing just makes sure a property doesn't exist.