[suggestion] New diagnostic: Use NullConstraint and EmptyConstraint for better assertion messages in case of failure

[Bartleby2718]

Continuing to file small tickets so that each ticket can be picked up by different people in parallel, if need be.

Both Assert.That(string.IsNullOrEmpty(myString)); and Assert.That(myString.IsNullOrEmpty()); could be converted to
Assert.That(myString, Is.Null.Or.Empty);.

Not sure what a good title for this diagnostic would be, though.

[mikkelbu]

I agree that we could add an analyzer with codefix for this