Improve repeatable UI

[JoryHogeveen]

Description

This PR aims to improve the repeatable fields UI.

1. Make it smaller and more integrated in the WP core UI elements.
2. Similar background and hover colors with WP core panels
3. Improve dragging (border handling)

Related GitHub issue(s)

Fixed #6853

Testing instructions

1. Create some repeatable fields
2. See current UI
3. Rebuild DFV with the changes in this PR
4. See new UI :)

Screenshots / screencast

Current (2.9.1):

Large fields:

Suggested:

Large fields: Hover & Draggins:

Changelog text for these changes

PR checklist

• [x] I have tested my own code to confirm it works as I intended.
• [x] My code follows the WordPress Coding Standards.
• [x] My code follows the WordPress Inline Documentation Standards.