Add a `not` expectation property returning an opposite expectation

[kylef]

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This allows you to run expectations such as:

try expect(...).to.not.beNil()

[ilyapuchka]

@kylef here I implemented my version of this that gives a bit better code completion options. With my implementation it's only possible to use to or to.not after expect(...) and then returned objects have their own sets of conditions. So that things like expect(...).beNil, expect(...).to.to or expect(...).to.not.to.to are not possible.