[UX] Search settings: Convert the "Search items" section to a table.

Related/follow-up to #2743 ...

Describe your issue or idea

The settings in this page are already overwhelming as they are, and if more search items were to be added to the list, it would get longer and longer, requiring too much scrolling.

To top it off this section specifically does some weird things with its validation as well in order to ensure that the default search item does not get disabled, introducing a really weird UI pattern:

I feel that if we converted this to the same UI pattern we use in the Language management UI, it would be much better...

Steps to reproduce (if reporting a bug)

Navigate to /admin/config/search/settings

Actual behavior (if reporting a bug)

Expected behavior (if reporting a bug)