LibWeb: Parse CSS fit-content(<length-percentage>) values

Before this change, we only parsed fit-content as a standalone keyword,
but CSS-SIZING-3 added it as a function as well. I don't know of
anything else in CSS that is overloaded like this, so it ends up looking
a little awkward in the implementation.

Note that a lot of code had already been prepped for fit-content values
to have an argument, we just weren't parsing it.

Libraries/LibWeb/Forward.h

@@ -176,6 +176,7 @@ class EdgeStyleValue;
 class ElementInlineCSSStyleDeclaration;
 class ExplicitGridTrack;
 class FilterValueListStyleValue;
+class FitContentStyleValue;
 class Flex;
 class FlexOrCalculated;
 class FlexStyleValue;