Update decode-audio-data to modern JS

[teoli2003]

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

This PR updates the decode-audio-data example:

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

In addition:

• Use now the unprefixed version of Web Audio API only (ubiquitous and prefixed versions are not supported by browsers anymore)
• Pass Prettier
• Fix the HTML code:
  • Declare the language
  • Fix the title
• Remove an unused variable
• Remove the comment it doesn't work in Safari: it does now.

This doesn't change the use of XHR to fetch.

Tested on Firefox, Safari, and Chrome (macOS) via a local server as it uses XHR.