Feature request: Base91 or Base92 encoder/decoder

**Is your feature request related to a problem? Please describe.**
When I was trying to decrypt some Base91 or Base92 encoded texts, I have to seek some other online sites to decode them. It is not so convenient. Hopefully CyberChef will add Base91 & Base92 encoder and decoder.

**Describe the solution you'd like**
Added encryption and decryption support for Base91 & Base92.

**Describe alternatives you've considered**
Other tools can encrypt or decrypt online, but not so convenient.

The issue names no files, tests, or entry points. Start by locating CyberChef's existing encoding and decoding operations and their tests, then determine how Base91 and Base92 should be represented as separate operations. Done means both formats can encode and decode correctly, with coverage for representative inputs and round-trip behavior.

Written by the indexing model from the issue text.