fix: extend colorOperand to cover all relative-color channel letters

[Copilot]

colorOperand had two bugs corrupting calc() expressions inside relative color syntax (rgb(from …), lab(from …), etc.).

Bug 1 — whitespace consumption dropped operators

/^[lchrgbs]\s+/ consumed the trailing space as part of the match. When a channel keyword appeared as the right-hand operand (e.g. calc(100 - r)), the parser swallowed the - operator silently:

rgb(from blue calc(100 - r) g b)  // emitted: calc(100  r)
rgb(from blue calc(r + 100) g b)  // threw: Could not parse call arguments or missing ')'

Fixed with a lookahead ((?=\s|[,/*)]|$)) and returning the captured letter (match[1]) rather than the full match.

Bug 2 — character class missing a and w

[lchrgbs] only covered RGB / HSL / LCH / OKLCH channels. a (Lab/OKLab) and w (HWB whiteness) were absent, causing identical parse failures:

lab(from blue l calc(a + 5) b)    // threw: Could not parse call arguments
hwb(from blue h calc(w + 10) b)   // threw: Could not parse call arguments

Full single-letter channel set across all CSS relative color functions — r g b (rgb), h s l (hsl), h w b (hwb), l a b (lab/oklab), l c h (lch/oklch) — unique: [abcghlrsw].

Tests

Added regression cases in modern.less / modern.css for both bugs: calc(100 - r), calc(r + 100), calc(a ± n) in lab(), and calc(w ± n) in hwb().

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