Add split and join functions to filterx

[jszigetvari]

It would be great to have a function to easily split a string along a delimiter character, and to get an array back, without having to use parse_csv for that.
Normally there also is a join function, that comes hand-in-hand with split. We should also add that to filterx.

[jszigetvari]

We must not forget to update the relevant application configs once these functions become available.