JSONata $base64decode does not support UTF8 strings

[gregorius]

@knolleary

not something we fix in Node-RED itself.

To avoid future confusion, one could remove the two functions from NR's JSONata support, i.e., that $base64encode and $base64decode cannot be accessed from within NR.

Technically speaking then there is something that NR could do but probably shouldn't do because of backward compatibility and also it does not seem to have bothered anyone until now anyway.

So this becomes more of an FYI for future "end" users.