Update offline-audio-context-promise to modern JS

[teoli2003]

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

This PR updates the offline-audio-context-promise 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
• Use promise versions of API if possible.
• Migrate XHR to fetch()
• Make sure the AudioContext ctor is called after user interaction.

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

[MrBrain295]

If you move the closing tag for the body element one line above the closing tag for the html element of fixes a Nu HTML checker error.

[teoli2003]

You're right, this is an oversight. The script should be in the header or the body. (ideally <script> in the header and the JS in a separate file, but this is significant work for little outcome).

Fix incoming.