Make islands' region system more generic

Problem/Motivation

We have a bug when creating a new Profile. Some View panels have no region set and they appear in the unexpected one (Main instead of Sidebar):

The logic may be wrong in ProfileForm:

      // If new, only library is on sidebar by default.
      // @todo move this position option to Island configuration.
      if ($id !== 'library' && !isset($configuration['region']) && isset($definition['enabled_by_default'])) {
        $region = IslandTypeViewDisplay::Main->value;
      }
      else {
        $region = $configuration['region'] ?? NULL;
      }

Proposed resolution

Instead of fixing the bug, it may be the opportunity of improving and future-proofing the logic.

All island types could have a region. We are using the feature only for View panels for now but, for example, we may want to use it for Buttons later (position buttons at the start are the end of the toolbar).

Also, it will help us to propose the Island plugin type to other projects: #3554417: Isolate islands logic to its own module

• Add a region (or default_region?) property in Island attributes.
• region is already a generic property of display_builder.islands.* 👍
• Set the default region to all View panels islands
• Move IslandTypeViewDisplay to a generic mechanism (a single IslandRegion? An enum per IslandType ?) Each Type can have a region enum or not, with a default region.

What else?