AK: Make it possible to not using AK classes into the global namespace

This patch adds the `USING_AK_GLOBALLY` macro which is enabled by
default, but can be overridden by build flags.

This is a step towards integrating Jakt and AK types.

AK/Array.h

@@ -111,5 +111,7 @@ constexpr static auto iota_array(T const offset = {})
 
 }
 
+#if USING_AK_GLOBALLY
 using AK::Array;
 using AK::iota_array;
+#endif