feat(v2): create droppable empty form placeholder

[mantariksh]

Currently the form builder renders an empty page when the form has no fields. This PR adds the placeholder for fields to be dragged in when the form is empty:

Moreover, previously, background colour of existing fields would change to neutral.100 when a new field was being dragged over, even though they are not responsive to new fields being dropped over them. This is inconsistent with the design system. This PR fixes the issue by preventing the background colour of existing fields from changing when new fields are being dragged over.

Other improvements

• Mocks added for POST request to create field and PUT request to update field

Known bugs

These will be fixed in a future PR:

• After creating a new field, the onSuccess handler in the Edit* component does not fire, because the component unmounts and re-mounts due to its key in the parent changing.
• After updating a field and exiting the edit field drawer, the field reverts back to its previous state. When attempting to create a new field, the field flips into the new state again.

[karrui]

LGTM