Packages: Date: Stop exposing Moment object on public interface

[aduth]

Discussed today in the Slack #core-js Weekly Chat (link requires registration)
Related: #9485

The public interface of all of our modules should serve as a contract to which we commit to forward compatibility. For this reason, we should limit surface area where possible, notably in implementation detail of underlying dependencies.

Task: For the packages/date module, we should remove all exposed methods which commit us to Moment.js . If necessary, this may require the addition of substitute interfaces. These should be implemented in a fashion non-specific to Moment and which are designed in a way that we are generally comfortable to committing to as a contract.

Extra Credit: Improve documentation of the packages/date module to document available public functions.

[aduth]

Closed by #11418 (cc @youknowriad)