trigger, irc: use STATUSMSG to handle status specific messages in channels

[Exirel]

Description

Fix #1886.

In this PR, I modified PreTrigger to parse the sender to check if the prefix was a STATUSMSG prefix, i.e. a message intended to a specific privilege level in the channel (such as ops, voice, etc.). Once this is done, the PreTrigger remove that prefix from the sender to create an instance of Identifier. On the bot's side, every time a PreTrigger is instantiated, bot.isupport.get('STATUSMSG') is provided.

I added some tests to show how that works and ensure it works as intended without breaking anything, so we should be all good.

Two notes to consider for the future:

• we may want to change the list of channel prefix to something more restrictive, or not
• we may want to set a default value for the STATUSMSG value, or have something a bit smarter about that

Both to be considered later, not in this PR.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

I don't think this version of the patch complies with the STATUSMSG spec. See line notes.

Don't worry, there are also some of the usual doc/comment nitpicks. 😜

[dgw]

I think it's squash time. Might be spaghetti squash, but time will tell us that. 🙃

[Exirel]

Squashed!