LibWeb: Parse the linear-gradient() CSS function

This should parse linear-gradient()s as defined in the W3 spec
https://drafts.csswg.org/css-images/#linear-gradients.

Note: This currently cannot parse multi-position color stops,
these are shown on MDN and work in Firefox and Chrome, though do
not seem to be defined in the spec.

See: https://developer.mozilla.org/en-US/docs/Web/CSS/gradient/linear-gradient#gradient_with_multi-position_color_stops

P.s. This also allows -webkit-linear-gradient for compatibility.

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -58,6 +58,7 @@ enum class FlexBasis {
     Auto,
 };
 
+// Note: The sides must be before the corners in this enum (as this order is used in parsing).
 enum class SideOrCorner {
     Top,
     Bottom,