Update step-sequencer to modern JS

[teoli2003]

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

This PR updates the step-sequencer example:

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

In addition

• Remove the antipattern: using <button>+ARIA instead of the semantic <input type="checkbox">
• 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

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