LibWeb: Implement a CSS::Number class

We have the same "number and is-integer flag" structure in several
places, so let's put that in a class.

Userland/Libraries/LibWeb/Forward.h

@@ -59,6 +59,7 @@ class MediaList;
 class MediaQuery;
 class MediaQueryList;
 class MediaQueryListEvent;
+class Number;
 class NumericStyleValue;
 class OverflowStyleValue;
 class Percentage;