Update panner mode to modern JS

[teoli2003]

This is part of our project to update code to a more modern JS syntax.

This PR updates the panner-node example:

• Use const and let where possible
• Use arrow functions where possible
• Use literal string where possible

In addition:

• Use now the unprefixed version of Web Audio API only (ubiquitous and prefixed versions are not supported by browsers anymore)
• Use constructors instead of factory methods
• Pass Prettier
• Fix the HTML code:
  • Declare the language
  • Fix the title
  • Improve instruction
• Use promise versions of API if possible.
• Use fetch() instead of XHR
• Fix a link to an http:// url to an https:// so it actually works in browsers (mixed content error)
• Replace .innerHTML properties with .textContent

Tested on Firefox, Safari, and Chrome (macOS) via a local server.

[MrBrain295]

The script tag needs to be moved to to the body or the head the pass the Nu HTML checker.

[teoli2003]

You're right, fixed.