Implement DeeplFreeTranslator

Related to https://github.com/translate-tools/linguist/issues/457

Request example:

```js
await fetch('https://deeplx.vercel.app/translate', {method: 'post', headers: {'content-type': 'application/json'}, body: JSON.stringify({"text": "have a try", "source_lang": "auto", "target_lang": "ja"})})
```