LibWeb: Support :active pseudo-class for hyperlinks, :focus possibly

Adds support for the :active pseudo-class for hyperlinks (<a> tags
only).

Also, since it was very similar to :focus and an element having a
focused state was already implemented, I went ahead and implemented
that pseudo-class too, although I cannot come up with a working
example to validate it.

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -43,6 +43,7 @@ public:
             Enabled,
             Checked,
             Not,
+            Active,
         };
         PseudoClass pseudo_class { PseudoClass::None };