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/Base64.h

@@ -22,5 +22,7 @@ namespace AK {
 [[nodiscard]] String encode_base64(ReadonlyBytes);
 }
 
+#if USING_AK_GLOBALLY
 using AK::decode_base64;
 using AK::encode_base64;
+#endif