Add support for setting checkbox values to true/false explicitly

e.g.

```js
await automator.set('I confirm that these details are accurate.').to(true); // or `false`
```

in lieu of

```js
await automator.click('I confirm that these details are accurate.');
```

which only _toggles_ the state of the checkbox but does not know if it is setting it to `true` (checked) or `false` (not checked).