UX improvements for custom fields editor

[orthagh]

Summary:

• fields now include their label inside -> reduce width usage in the viewport
• we remove the preview of the field nature (input/dropdown)
• we keep preview of some attributes (mandatory, full width)
• a side panel helps retrieve all fields and drop them into the viewport
• a separate section permits the creation of custom fields
• native and custom fields are clearly identified
• A search input filters available fields
• We have 2 states for a field in the viewport: default and selected
• We show the controls for editing or hiding the fields only for selected state
• grip for drag&drop is always shown (to be discussed, we may have the selected state also)

Note, it's a proposed design, If you have suggestions, time to make them is before starting dev.

Current design: https://design.penpot.app/#/view/7ad540b5-8190-815d-8005-57c26f8b15dd?page-id=7ad540b5-8190-815d-8005-57c26f8b15de&section=interactions&index=0&share-id=a0a8e792-b2d2-818e-8005-57d3e4d4a58d