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/StringView.h

@@ -357,5 +357,7 @@ struct CaseInsensitiveStringViewTraits : public Traits<StringView> {
     return AK::StringView(cstring, length);
 }
 
+#if USING_AK_GLOBALLY
 using AK::CaseInsensitiveStringViewTraits;
 using AK::StringView;
+#endif