[D8] Introduce a new example.local.settings.php file

Didn't want to hijack the https://github.com/backdrop/backdrop-issues/issues/1855, because that one's been created for Documentation purpose, so am creating a new one.

Description of the need

As developers we find it always handy to set number of configurations in certain ways, for example, making it show all the errors on the screen, disabling some cache, or setting the current theme to rebuild its registry on every page load, etc. And instead of going through different pages on UI every time we start a new project, it's much easier just to use the feature discussed and introduced on https://github.com/backdrop/backdrop-issues/issues/1752.

Proposed solution

Nevertheless, the check for local settings file has already been introduced, manually compiling that kind of local settings file also takes valuable time. I suggest to take this further and add missing feature parity with Drupal 8 - as we can see on https://git.drupalcode.org/project/drupal/tree/8.8.x/sites Drupal 8 is shipped with the well-formatted example.settings.local.php file. It would be really useful for developers to have ready-to-use example.settings.local.php file in Backdrop core.