Obsidian: Convert all types of checkboxes

[sojusnik]

Zim has three additional states of checkboxes that Obsidian isn't supporting, but they are still rendered as checked off checkboxes, but without strikethrough text, so they are easily distinguishable from a regular checked off checkbox. Therefore, I suggest converting them too.

Zim (now):

[x] cross
[>] arrow right
[<] arrow left

Obsidian (now):

- [x] cross
- [>] arrow right
[<] arrow left

Obsidian (should):

- [c] cross (because a small x is already used for a checked off checkbox)
- [>] arrow right
- [<] arrow left

The following two types are already converted properly:

- [x] checked off
- [ ] not checked off