LibWeb: Implement IntegerStyleValue, for holding <integer>

Having one StyleValue for `<number>` and `<integer>` is making user code
more complicated than it needs to be. We know based on the property
being parsed, whether it wants a `<number>` or an `<integer>`, so we
can use separate StyleValue types for these.

Userland/Libraries/LibWeb/Forward.h

@@ -117,6 +117,7 @@ class IdentifierStyleValue;
 class ImageStyleValue;
 class InheritStyleValue;
 class InitialStyleValue;
+class IntegerStyleValue;
 class Length;
 class LengthBox;
 class LengthOrCalculated;