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/CSS/MediaQuery.h

@@ -213,7 +213,7 @@ private:
 };
 
 class MediaQuery : public RefCounted<MediaQuery> {
-    friend class Parser;
+    friend class Parser::Parser;
 
 public:
     ~MediaQuery() = default;