How to insert a carriage return and line feed into a variable using javascript

The simplest way is to just add <br> into the string where you want it to break. You can do this directly in Storyline when assigning the variable value.

Storyline accomodates some very basic formatting directly:

https://access.articulate.com/support/article/Storyline-360-Using-HTML-Formatting-Tags-in-Text-Variables 

For more control, you can insert escape characters into the string in JavaScript:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#escape_sequences 

Write the modified string back to a SL in the usual way using SetVar().

Keep in mind that SL textboxes will not automatically resize to accomodate dynamic content in variables. Make your text box the desired size and set to Do not autofit.