Enable JavaScript code autocomplete using `jsconfig.json`

[colinrotherham]

For recent contributions I've been running with a jsconfig.json file

It's helped me find and fix a few things but there's no necessity to merge

Code autocomplete

This PR will automatically pick up JSDoc annotations in IntelliJ, WebStorm, Visual Studio, Visual Studio Code and Sublime Text (via the TypeScript plugin) so you can see tooltips like:

Code checking

Optionally you can enable "checkJs": true when needed as a good way to spot bugs

  {
    "compilerOptions": {
+    "checkJs": true,
      "module": "NodeNext",
      "resolveJsonModule": true
    }
  }