Link module: Provide an option to allow for a predefined list of static titles

I'm splitting this from https://github.com/backdrop/backdrop-issues/issues/4055 as this is a new feature, so we may want to push to a minor release (I personally have no opinion on that).

This is the respective of Issue #1335424: Title as a select, which was introduced in the 7.x version of the Link module with https://git.drupalcode.org/project/link/commit/0235032

As the "static" Title, I'm proposing a title where the user can select the value from a select dropdown. This is useful when admins want to limit the Title to several values. Eg. Home, Office, School, Other.

Admin form for field:

Entity field widget: