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/Endian.h

@@ -145,6 +145,8 @@ requires(HasFormatter<T>) struct Formatter<BigEndian<T>> : Formatter<T> {
 
 }
 
+#if USING_AK_GLOBALLY
 using AK::BigEndian;
 using AK::LittleEndian;
 using AK::NetworkOrdered;
+#endif