getApexString() - Takes a string and returns the corresponding Apex (Salesforce) string. If string ends up being too long for one line it will be shown as a multiple line expression using String.join().

getApexString.js

function getApexString(string, maxOneLinerLength=80) {
    var normal = "'" + JSON.stringify(string).slice(1, -1).replace(/\\"/g, '"').replace(/'/g, "\\'") + "'";
    return (normal.length > maxOneLinerLength && string.indexOf('\n') >= 0)
        ? "String.join(new String[] {\n\t" + normal.replace(/\\[^n]|(\\n)/g, (m, isNewline) => isNewline ? "',\n\t'" : m) + "\n}, '\\n')"
        : normal;
}

~io-config.jsonc

{
  // Where the JS code lies
  "files": ["getApexString.js"],
  // Name of the function responsable for transforming the input into the output.
  "transform": "getApexString",
  // Arguments that will be passed to the function by name.
  "params": [
    {
      "name": "maxOneLinerLength",
      "label": "Maximum Length of a One-liner",
      "type": "number",
      "min": 3,
      "step": 1,
      "value": 80
    }
  ],
  // Input setup
  "input": { "language": "text" },
  // Output setup
  "output": { "language": "apex" },
  // Type of YourJS IO-App
  "type": "split"
}

Here is the corresponding IO App:
https://yourjs.com/gist-io/1db1f19c0b9e676fd1638ddad9c93e99