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/CMakeLists.txt

@@ -230,7 +230,7 @@ set(SOURCES
     StylePropertiesModel.cpp
     UIEvents/EventNames.cpp
     UIEvents/MouseEvent.cpp
-    URLEncoder.cpp
+    URL/URL.cpp
     WebAssembly/WebAssemblyInstanceConstructor.cpp
     WebAssembly/WebAssemblyInstanceObject.cpp
     WebAssembly/WebAssemblyInstanceObjectPrototype.cpp