✅ Parse tasklists to an AST

[rowanc1]

This parses tasklists into an AST, previously they were going in as HTML. This adds a checked property to the AST.

[fwkoch]

Nice, definitely better than leaving the HTML. It's a pretty gross path to get here... markdown -> tokens with "task list" classes -> child html for the checkboxes -> regex on that html to determine attributes to lift up to the list item -> delete html and fix up list item text. This is a case where working at the AST level is more complex than if we just modified the parsing logic itself - but that logic isn't very accessible.

Anyway, this works great, and the test cases are there!

[rowanc1]

Yeah, I looked at unified here and .. it is just super easy. :)

I think that the test cases are the most important here for catching things if we change in the future!