LibWeb: Add PercentageStyleValue, and parse it

This is in a slightly weird state, where Percentages are sometimes
Lengths and sometimes not, which I will be cleaning up in subsequent
commits, in an attempt not to change all of LibWeb in one go. :^)

Userland/Libraries/LibWeb/Forward.h

@@ -55,6 +55,7 @@ class MediaQueryListEvent;
 class NumericStyleValue;
 class OverflowStyleValue;
 class Percentage;
+class PercentageStyleValue;
 class PositionStyleValue;
 class PropertyOwningCSSStyleDeclaration;
 class Screen;