ob-js: Fix passing multiline variables

ob-js: Fix passing multiline variables

* lisp/ob-js.el (org-babel-js-var-to-js): Replace newline characters
  with "\n" in strings.

Let's say I have a multi-line string stored in an example block.

I want to store my CSV in an example block.

  ColA,ColB,ColC
  1,2,3
  4,5,6

I have a JavaScript function that accepts a string named 'csv' and passing in 'my-csv-data'.

  console.log(csv);

When I expand the source block I end up with:

var csv="ColA,ColB,ColC
  1,2,3
  4,5,6";
console.log(csv);

This will not execute correctly because JavaScript does not support newlines in strings.

What I want instead is:

  var csv="ColA,ColB,ColC\n  1,2,3\n  4,5,6";
  console.log(csv);

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