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/HTML/Scripting/Script.cpp

@@ -8,7 +8,7 @@
 
 namespace Web::HTML {
 
-Script::Script(URL base_url, String filename)
+Script::Script(AK::URL base_url, String filename)
     : m_base_url(move(base_url))
     , m_filename(move(filename))
 {