gh-37599: extend the definition of q-binomial coefficients to (k > 0, n < 0)
This is extending the domain of the q-binomial coefficients.
This is done by polynomiality wrt n, for k >= 0.
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/37599
Reported by: Frédéric Chapoton
Reviewer(s): Travis Scrimshaw
