GFM Lists Additions (Task List Items, Textarea Preview, Other Delimiters)

I've been developing the feature I've suggested in #2339 that is checkbox lists or task list items. This feature adds the markdown support for task list items in the form of The ability to create task list items is limited to a parser flag, meaning that this is configurable. I did not end up adding automatic message editing as I figured that the editing can just be done normally, with this featuring being only a client-side rendering update. You can currently test the feature at https://squll.fartcore.ai/ and the code is at https://github.com/HellishBro/fluxer/