Port Views Distinct module to Drupal 8

Related: https://www.drupal.org/project/views_distinct/issues/3384098 ✨ Drupal 10 Compatability Active

Also related: https://www.drupal.org/project/drupal/issues/2993688 🐛 Views relationships with multi-valued entity reference fields invalidate Distinct query option Needs work

I just now used the version from #36 on a D10 website. Worked fine. Here's the contents of the .info file:

name: Views Distinct
type: module
description: Allows filtering "distinct" Views result rows based on arbitrary fields.
package: Views
core_version_requirement: ^8.8 || ^9 || ^10
dependencies:
  - drupal:views