LibWeb: Move CSS Parser into new Web::CSS::Parser namespace

The goal here is to move the parser-internal classes into this namespace
so they can have more convenient names without causing collisions. The
Parser itself won't collide, and would be more convenient to just
remain `CSS::Parser`, but having a namespace and a class with the same
name makes C++ unhappy.

Userland/Libraries/LibWeb/Forward.h

@@ -98,6 +98,10 @@ enum class PropertyID;
 enum class ValueID;
 }
 
+namespace Web::CSS::Parser {
+class Parser;
+}
+
 namespace Web::DOM {
 class AbstractRange;
 class AbortController;