Read a light-dark() color as its light color - #546

A light-dark() color is not read, so the shape loses its fill or stroke and svglib logs "Can't handle color". draw.io writes light-dark() in its SVG exports, usually in a style that overrides a plain color attribute, for example stroke="#63de17" style="stroke: light-dark(rgb(99, 222, 23), rgb(237, 237, 237))" in the icons of the Zensical theme used by Python-Markdown's documentation (.zensical/theme/assets/images/pyMd_icon.svg): the icon's outline disappears.

In CSS Color 5, light-dark() uses its first color unless a color-scheme asks for dark, and SVG documents rarely set one, so convertColor() now converts the first argument. A new _first_css_argument() finds it, skipping the commas inside rgb(...) or hsl(...). With this change the outline of that icon is green, as in a browser.

Added test_light_dark_color (rgb, a different case, hsl, a single argument, and currentColor); it fails on main. Updated tests/public_api.json with --update for the new private name, and added a ### Fixed entry in CHANGELOG.md (I used #547, I'll fix it if the PR gets another number). pytest (without the network-dependent sample downloads), ruff format --check, ruff check and mypy src tests pass.

AI-assisted (Claude Code); I reviewed and tested the change