Add a theme_dev_mode toggle via scaffolded files

[deviantintegral]

While Drupal now has better features in this area (https://www.drupal.org/project/drupal/issues/3278493), it doesn't address all of the tweaks that a developer might want to make to settings when actively developing on the front-end. Let's set up some additional files to be scaffolded into the sites directory to support this.

Requirements

• The default behaviour must not affect existing sites that are upgrading drainpipe - meaning, we don't default anything to "on" by virtue of IS_DDEV or anything like that.
• By default, debugging is off and caches are on. This follows a best practice of normally developing and testing with caching enabled, and selectively disabling it when needed.
• Do not require a ddev restart to change the mode. This means no use of ddev config - at most, .env settings or settings.php settings.
• When theme dev mode is on:
  • Turn on twig debugging
  • Enable the null cache bin
  • Disable the theme render and related caches
  • Enable cache tags in http response headers
  • Stepping through twig templates in IDEs should work