JsString.encode() escape characters

[ambrusadrianz]

with (tEncoder) { T.encode() } outputs invalid Json if the value contains a multiline string/string with quotes/etc.

Escaping of special characters should be done in the JsString encoder. This is a pretty major issue, I think. I'm eager to help with a PR if needed.

[raulraja]

@ambrusadrianz please feel free to send a PR with your suggestion. Thanks!