Support two-position gradient color stops

[jhaygood86]

Problem

A gradient color stop with two positions is rejected, taking the whole gradient with it:

linear-gradient(red 0 50%, blue 50% 100%)   /* dropped */
linear-gradient(90deg, red 0 8px, blue)     /* dropped */
radial-gradient(red 0 8px, blue)            /* dropped */

CSS Images 4 §3.5.1 defines a stop's length as:

<linear-color-stop> = <color> <color-stop-length>?
<color-stop-length> = <length-percentage>{1,2}

with the note that two positions is "equivalent to specifying two color stops with the same color, one for each position" — a convenient way to make solid bands.

GradientConverter.ToGradientStop consumes only a single trailing <length-percentage>, so a stop written with two leaves a leftover token, ToGradientStop returns null, and ToGradientStops fails the entire value.

Fix

Parse a second position when one is present, and keep both in source order so the stop round-trips as authored — red 0 50% serializes back as red 0 50%, not red 50%. StopValue now holds an optional second position and joins whatever is present.

Guards from the grammar:

• the two-position form requires a color — a bare <length-percentage> is a single-position <linear-color-hint>, so 0 50% with no color stays invalid
• three positions (red 0 25% 50%) stay invalid

The radial case needed no separate change. ConvertFirstArgument already returns null for a comma group that begins with a <color>, so red 0 8px was always routed to the first stop — it simply couldn't be parsed until now. Confirmed by radial-gradient(red 0 8px, blue) passing with only this change.

Tests

9 cases in Gradient.cs: three two-position gradients (linear, linear-with-angle, radial) asserting the exact serialized value; three single-position/no-position/hint cases that must stay legal; and two malformed cases (three positions, two bare positions) that must stay rejected.

3 fail on master. The full suite (1263 existing tests) stays green with no existing assertion modified, and all seven target frameworks build with no new warnings.