Fix 431 triple slash comments

**TODO: add to documentation**

---

This commit adds the ability to comment code using both `//` (Ctrl+/) and `///` (Ctrl+Shift+/).

To use `///` comments, however, it is first required to copy this key binding to *Packages/User/Default.sublime-keymap*:

```json
	{
		"keys": ["ctrl+shift+/"], "command": "dart_toggle_comment", "args": { "block": true },
		"context": [
			{ "key": "selector", "operator": "equal", "operand": "source.dart", "match_all": true },
			{ "key": "setting.command_mode", "operand": false }
			]
	},
```

Otherwise, ST will keep using the default keybinding located in *{Data Path}/Packages/Default*, as it has a higher precedence due to alphabetical order.