NUnit2010: Add support for detecting use of 'is' pattern inside 'Assert.That'

[manfred-brands]

Fixes #691

[manfred-brands]

@mikkelbu I have added this to the EqualConstraintUsageAnalyzer because it started as is 3.
Because I had fun, it actually also converts relational constraints like Is > 1 and < 2.
So it could also fit in the ComparisonConstraintUsageAnalyzer.

One alternative is to make a separate analyzer, but that is a lot more work.

[mikkelbu]

Looks good @manfred-brands, and I think it is fine to keep it within the existing analyzer.

My only minimal concern is that the And/OrConstraints does not exactly match the semantics of the patterns. For instance, we have the note in https://docs.nunit.org/articles/nunit/writing-tests/constraints/OrConstraint.html#evaluation-order-and-precedence, but one might argue that this failure in the OrConstraint is an error and an implementation detail.

I've also tried to test some more complicated scenarios - combining multiple And and Ors - as I thought I could remember that we also had some problems with the precedence in NUnit, but I was not able to find the problem (if there is one).

So all in all I think the PR is an improvement and I'm not too worried about the differences as I don't think most people have very complicated patterns in their asserts

[manfred-brands]

Thanks @mikkelbu