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/Hex.h

@@ -39,6 +39,8 @@ String encode_hex(ReadonlyBytes);
 
 }
 
+#if USING_AK_GLOBALLY
 using AK::decode_hex;
 using AK::decode_hex_digit;
 using AK::encode_hex;
+#endif