Related to previous question, how can we format string escaped value?

Question

Related to the previous question How to make _source field dynamic ? I was able to make search template _source field dynamic from the front-end, but due to invalid JSON format, I had to make it to string format. which is very hard to read. Is there any way to make it in a readable form? I tried \ after each new line to make (as suggested in ruby) but could not get it working.

"source": "{\"query\":{\"bool\":{\"must\":{\"match\":{\"line\":\"{{text}}\"}},\"filter\":{{{#line_no}}\"range\":{\"line_no\":{{{#start}}\"gte\":\"{{start}}\"{{#end}},{{/end}}{{/start}}{{#end}}\"lte\":\"{{end}}\"{{/end}}}}{{/line_no}}}}}}"

this is the string query which saved in a YML file.

I tried with ruby multiline string but still giving a parsing error. I have created a template.yml file and store the template as given below

template: |
{
    "script": {
        "lang": "mustache",
        "source": '{'\
            '"_source": {{#toJson}}fields{{/toJson}}'\
        '}'\
    }
}

also tried the replace with double quotes and still backtick not helping.