Recognize promise then call with rejection handler as valid floating promise handling

[vilicvane]

PR checklist

• Addresses an existing issue: Allow then(fulfillment, rejection) for no-floating-promises #2879
• enhancement
  • Includes tests
• Documentation update

Overview of change:

createPromise()
  .then(() => next(), next); // No longer an error when rule `no-floating-promise` is enabled.

[bugfix] no-floating-promises: recognize rejection handler passed as second argument to promise.then()

[palantirtech]

Thanks for your interest in palantir/tslint, @vilic! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[ajafff]

The code changes look good, but this definitely needs a test.

[vilicvane]

@ajafff Test added~ :)

[ajafff]

Thanks @vilic