Update spatialization to modern JS

[teoli2003]

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

This PR updates the spatialization 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
• Rename spacialisation to spatialization, even in file and directory names
• Fix README.md to pass Markdownlint.

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