Use Tab to indent Todo list level

[cvn]

Describe your feature request

I'd like if Todo lists could be indented/unindented with the Tab key. This functionality was recently added for ordered and unordered lists in 6.5.0. This was an excellent addition, which I'm very thankful for. It would great if Todo lists worked the same way.

Additional context

The discussion in #627 about why this is valuable for lists also applies here.