Add An+B serialization alongside parse_nth

[ScribleSean]

Closes #56.

Adds tinycss2.nth.serialize_nth(a, b) following the CSS Syntax An+B serialization algorithm. This gives callers the inverse of parse_nth, including canonical handling of zero coefficients and the omitted coefficient in n and -n. The function is documented in the existing An+B API section.

Includes canonical-output and parser round-trip tests. The full suite passes (17,022 tests) on macOS/Python3.14, along with Ruff and the Sphinx documentation build. Other platforms were not tested locally.

Prepared with Codex (GPT-6 family; exact serving variant was not recorded) assistance and an Antigravity gemini-3.1-pro-high candidate assessment. I personally reviewed and approved this work before submission.

[liZe]

Thanks for your pull request.

So, you used both GPT-6 and Gemini Pro to transform this:

into:

I appreciate your honesty, but I guess your brain would have probably been enough… or actually better. 😁

I changed a couple of things on top of your pull request. And I learned there was a sign format specification for f-strings on the way.

Philosophical questions of #77 (comment) still apply.