LibWeb: Add CSS::Size to represent the full range of CSS size values

Until now, we've been using CSS::LengthPercentage, sometimes wrapped in
Optional, to represent CSS sizes.

This meant we could not support modern values like `min-content`,
`max-content`, `fit-content(<length>)`. We were also conflating `none`
and `auto` which made the `min-*` and `max-*` properties confusing.

The new CSS::Size class covers all possible size values as individual
substates. It'll be quite a bit of work to make all layout code aware of
the additional features, this patch merely makes the new type available.

Userland/Libraries/LibWeb/Forward.h

@@ -91,6 +91,7 @@ class ResolutionStyleValue;
 class Screen;
 class Selector;
 class ShadowStyleValue;
+class Size;
 class StringStyleValue;
 class StyleComputer;
 class StyleProperties;