Operation request: Some new codes

Does the author consider adding the base91 decoding function?
I always use CyberChef as an AIO(all in one) tool，Although it is not as common as other bases, as an "AIO" tool, the more comprehensive the better，Similar to xxencode and aaencode, I think they will be used more in CTF competitions

Start by locating the existing xxencode and aaencode operations and the tests or registration points associated with them. Compare how those operations define their decoding behavior, then determine the expected Base91 input and output cases; done means a Base91 decoding operation is available and covered by matching tests.

Written by the indexing model from the issue text.