Add a highlights 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

Chatterino's highlight system isn't scriptable at all.

I don't have a concrete idea on how this should look but I'd say Lua should somehow receive an event when Chatterino checks for highlights and Lua should return which highlight to use. Maybe even the same system as #4624

Screenshots

N/A

OS and Chatterino Version

Chatterino 2.4.3 DEBUG (commit 1f0aa80 modified) built with Qt 5.15.9 Running on Arch Linux, kernel: 6.3.1-arch1-1