Register ESLint as an available language server (#11178)

This PR adds the ability for the ESLint language server (`eslint`) to be
controlled by the `language_servers` setting.

Now in your settings you can indicate that the ESLint language server
should be used for a given language, even if that language does not have
the ESLint language server registered for it already:

```json
{
  "languages": {
    "My Language": {
      "language_servers": ["eslint", "..."]
    }
  }
}
```

Release Notes:

- N/A