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/StringImpl.h

@@ -119,6 +119,8 @@ struct Formatter<StringImpl> : Formatter<StringView> {
 
 }
 
+#if USING_AK_GLOBALLY
 using AK::Chomp;
 using AK::NoChomp;
 using AK::StringImpl;
+#endif