Allow multiple default values on multi value fields

Describe your issue or idea

Currently, the default value field when editing a field does not support having more than one default value. It would be nice to have the ability to add more than one default value in some cases.

From my testing, it appears that most of the internal functionality for this is already in Backdrop, and all we need to do is modify the form for editing field settings.

Based on the Drupal core issue here: https://www.drupal.org/project/drupal/issues/1482386.

PR by @hosef: https://github.com/backdrop/backdrop/pull/2349