LibWeb: Add the Web::URL namespace and move URLEncoder to it

This namespace will be used for all interfaces defined in the URL
specification, like URL and URLSearchParams.

This has the unfortunate side-effect of requiring us to use the fully
qualified AK::URL name whenever we want to refer to the AK class, so
this commit also fixes all such references.

Userland/Libraries/LibWeb/CSS/StyleValue.cpp

@@ -152,7 +152,7 @@ Color IdentifierStyleValue::to_color(const DOM::Document& document) const
     }
 }
 
-ImageStyleValue::ImageStyleValue(const URL& url, DOM::Document& document)
+ImageStyleValue::ImageStyleValue(const AK::URL& url, DOM::Document& document)
     : StyleValue(Type::Image)
     , m_url(url)
     , m_document(document)