feat(assertions): add 'still' language chain

[ScottRudiger]

To increase readability it's sometimes desirable to have a still language chain. Take this simplified test for example:

it('should have a length property that does not decrease below zero with excessive popping', () => {
  const array = [1, 2];
  array.pop();
  expect(array).to.have.lengthOf(1);
  array.pop();
  expect(array).to.have.lengthOf(0);
  array.pop();
  expect(array).to.still.have.lengthOf(0);
  array.pop();
  array.pop();
  array.pop();
  expect(array).to.still.have.lengthOf(0);
});

For my use case, it was easy enough to add an in-file helper with the following:

chai.use(({Assertion}) => Assertion.addProperty('still'));

But I thought I'd throw in a quick PR in case the maintainers agree this fits with the scope of the project.

Let me know if you'd like me to bump the version for a minor release and I'll add it to the PR (I'm familiar w/ the contributing guidelines).

I'll happily submit a corresponding PR to update the docs if this PR is accepted.

[meeber]

I'm okay with adding still as a language chain. I don't think there's anything else that would need to be changed in terms of docs. A couple of tests would need to be updated for expect and should interfaces.

@chaijs/chai: Any strong feelings one way or the other about this?

[ScottRudiger]

A couple of tests would need to be updated for expect and should interfaces.

@meeber I incorporated still into the tests for language chains in test/expect.js and test/should.js. Please let me know if there's something else I missed.

[codecov]

Codecov Report

Merging #1194 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1194   +/-   ##
=======================================
  Coverage   94.51%   94.51%           
=======================================
  Files          32       32           
  Lines        1676     1676           
  Branches      404      404           
=======================================
  Hits         1584     1584           
  Misses         92       92

Impacted Files	Coverage Δ
lib/chai/core/assertions.js	99.39% <100%> (ø)	⬆️

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