[9.x] Searchable authorization

[duncanmcclean]

This pull request fixes an issue where Runway models in the Statamic CP command palette search would fail authorization and then crash.

There were two separate problems:

• Runway searchables failed authorization checks
  • This was happening because Statamic CP search filters results using User::current()->can('view', $item->getSearchable()), but the Searchable class didn't have a policy registered.
  • I've fixed it by adding a SearchablePolicy that delegates to the existing ResourcePolicy.
• Runway searchables crashed due to missing getCpSearchResultIcon() method
  • This was happening because Statamic's CP search result rendering expects this method.
  • I've fixed it by adding the method to the Searchable class.

Fixes #796

[github-actions]

Released as part of v9.3.2.