Add a proper Channels API

[Mm2PL]

Checklist

• I'm reporting a problem with Chatterino
• I've verified that I'm running the most recent nightly build or stable release
• I've looked for my problem on the wiki
• I've searched the issues and pull requests for similar looking reports

Describe your issue

Currently the channels Lua API looks like this:

c2.send_msg(
  channel_name, -- string
  message -- string
) -- -> bool
c2.system_msg(
  channel_name, -- string
  message -- string
) -- -> bool

This has some issues:

• it is not possible to check if a channel is open without sending something to it
• it is not possible to use IRC channels
• it might have issues with /watching? I don't know exactly as I don't use Windows and can't test with /watching.

I'd like something more like getOrAddChannel method of TwitchIrcServer. However such a system has a big problem: should Lua holding references still hold the channel open? If yes, the C++ part will probably be quite complex with massaging std::shared_ptrs to work with Lua's GC. Otherwise Lua scripts will be very complex with checking if the channel still exists.

Screenshots

No response

OS and Chatterino Version

Chatterino 2.4.3 DEBUG (commit bcd8028) built with Qt 5.15.9

[Mm2PL]

This was done in #5141