When the host page takes control of side-by-side mode by adding:

```
sideBySide: { mode: 'manual' }
```

To the client's config, the client needs a way to determine whether side-by-side
mode was applied when the user clicks in the document. If it is applied, the
sidebar should remain open, otherwise it should be closed.

This PR adds an `isActive` callback to the `sideBySide` settings which the
client will invoke when it needs to know if side-by-side is active or not.