Toolbar button to add a task list

There should be a toolbar button to add a task list.

```markdown
# Groceries
- [ ] Apples
- [x] Bananas
- [ ] Oranges
```

FontAwesome icon: `fa-tasks`
https://fontawesome.com/v5/icons/tasks?s=solid

Start by locating the toolbar button definitions and the editor action that inserts Markdown. Use the provided task-list example and the FontAwesome fa-tasks reference to define the button; done means the toolbar inserts the expected unchecked task-list markup.

Written by the indexing model from the issue text.