jsonnetfmt: convert strings with `\n` to use ||| multiline

[ghostsquad]

This is a feature request (or at least an idea) to be able to detect & convert strings with \n in them to use the ||| multiline format.

[alexhulbert]

Hacky regex I wrote to do this. Probably has some edge cases I didn't think of but its better than editing them by hand:

https://gist.github.com/alexhulbert/738d830399bc05242a3a71569313c844