When a user pastes JSON into the editor, the editor prompts to convert it to YAML. This works if the JSON contains leading spaces:
```
  	 {"foo": "bar"}
```
but does not currently work if the leading whitespace includes new line characters or tabs -- in this case JSON is pasted "as is".
```


{"foo": "bar"}
```

This PR improves the `isJSON()` function to recognize `\t`, `\r` and `\n` in the leading whitespace. Tabs and new lines are allowed in the leading whitespace according to [RFC 4627](https://tools.ietf.org/html/rfc4627) (section 2) and [ECMA-404](https://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf) (section 4):
> Insignificant whitespace is allowed before or after any of the six structural characters.
> 
> The whitespace characters are: character tabulation (U+0009), line feed (U+000A), carriage return (U+000D), and space (U+0020).