Support Custom Checkboxes

[luposmi]

Idea is pretty simple, support custom checkboxes aka don't only have [x] but also, e.g., [!] for important.
Here are some links to implementations in Obsidian (not an official feature, but supported by custom themes)

• custom theme "minimal"
• forum post
• some random github readme idk

just wanted to bring this up now as the #185 custom callouts are design wise a pretty similar problem, and it may be possible to do both at once (at least conceptually - replacing regex with svg and text + color-, idk if they are handled differently internally)