[BUG] `typeof navigator !== 'undefined'`, but `navigator.userAgent` is undefined

Within Deno Worker

```js
  self.postMessage([null, typeof navigator])
  self.postMessage([null, typeof navigator.userAgent])
```

```js
[ null, "object" ]
[ null, "undefined" ]
```

So, `navigator.userAgent.includes` throw error.

Related

https://github.com/flexdinesh/browser-or-node/pull/25