This PR changes the Markdown language defaults to set `format_on_save`
to be `off`.

Prettier's Markdown formatting is a bit controversial for some people,
so we turn it off by default.

To restore the previous behavior, add the following to your settings:

```json
{
  "languages": {
    "Markdown": {
      "format_on_save": "on"
    }
  }
}
```


Release Notes:

- Changed the default `format_on_save` behavior for Markdown files to be
`off`.