Generate valid JS for multi-line setting

[renatgalimov]

In v1.0.4, if you have a multi-line string in settings, it will generate an invalid JS.

We use serde_json to ensure that the content we build it a valid JSON. We use a JS parser to check the generated JS in tests.

Before this fix it would generate multi-line settings as:

   settings: {
       "enable_analytics": "true",
       "key": "value",
       "override_timezone": "",
       "tag_manager_id": "",
       "multi_line": "multi
line
string"
   },

How has this been tested?

1. Update the test case and ensured that the test is faliing
2. Implemented the fix.

Checklist before merging

• If have updated tests.
• Is this a new feature? No.
• I've attached relevant screenshots (if relevant). No