Todos should be cancelable

Todos need to have a "canceled" state so that they can be cleaned out of "open" lists without being "done". This will make managing/cleaning up old task lists in queries much easier.

Looking around at other prior art on the subject:

A todo-md "Standard" uses [-] as a "declined" option.
The "Tasks" plugin for Obsidian details these custom task statuses and also mentions [-] as "canceled", thought without explicitly saying it's standard. It also mentions using [/] as "partially complete". This is a neat idea. Neat enough to avoid using that character as "canceled".

To implement, will need:

New parsing & index data.
New icons and css.
New query support (i.e. Canceled Tasks). Will probably also want to differentiate Done and Complete, as "Canceled" falls into the first, but not the second (or vice-versa depending on your mood).

suchnsuch / Tangent

Todos should be cancelable #46