[1.0.0-alpha2] Add "content edit" panel

Problem/Motivation

Display Builder is not a content editor and doesn't replace the content entity edit form.

However, similar solution are providing this feature and it seems easy to add.

Proposed resolution

Add a new island panel:

• With the unaltered content entity edit form
• Every change save the entity
• Only displayed when content edit context (so only in entity display overrides)
• Only displayed in default and full displays (default and full are often the same but not always) ?
• What would be the settings of this panel? (once ✨ [1.0.0-alpha1] Make Island plugins configurable Active is ready)