[trigger.pl] Add support for fish encrypted channel messages and PRIVMSGs

This issue is confirmed with irssi-fish from repository https://download.opensuse.org/repositories/home:/ailin_nemui:/irssi-git-an/xUbuntu_22.04/amd64/ and possibly other FiSH plug-ins

Despite fish encryption/decryption working while the FiSH module is loaded in Irssi, trigger.pl does not currently read FiSH decrypted messages in channel. For example, consider the following message in #irssi on Libera:

> This is a FiSH decrypted message

trigger.pl sees this message in its encrypted format which looks something like this:

+OK YRAtW0aJTWs0FV2ij/pXiSp/X0..k.95Izl1FwABA/LJ0x60

So unless the trigger is set to some variation of the above string, the trigger will never work.

Start by reproducing the issue with Irssi and irssi-fish, then inspect trigger.pl's handling of channel messages and PRIVMSGs. Done means trigger.pl can match FiSH-decrypted message content instead of only the +OK encrypted form.

Written by the indexing model from the issue text.