Add support for offline writes

This bumps up the libsql package dependency and adds a "offline" flag to
config, which enables offline writes with sync to remote:

Configuration is straight-forward:

```
const config = {
    url: process.env.URL ?? "file:local.db",
    syncUrl: process.env.SYNC_URL,
    authToken: process.env.AUTH_TOKEN,
    offline: true,
};
const db = createClient(config);
```

This puts the database in a mode where writes are always local and
`sync()` pushes changed frames to remote server.