LibWeb: Implement linear-gradient() transition hints

These allow you to specify the point were the gradient transitions
from one color to the next (without a transition hint the transition
occurs at the point 50% of the way between the two colors).

There is a little bit of guesswork in this implementation as the
specification left out how hints work with the color stop fixup,
though it appears that they are treated the same as color stops.

Userland/Libraries/LibWeb/Painting/GradientPainting.h

@@ -17,6 +17,7 @@ namespace Web::Painting {
 struct ColorStop {
     Gfx::Color color;
     float position = 0;
+    Optional<float> transition_hint = {};
 };
 
 using ColorStopList = Vector<ColorStop, 4>;