Add a "Click to edit" feature in Live preview

[ccapndave]

When developing websites with big and complicated pages, it would be extremely useful to be able to click on something in the live preview and have an associated field be focused in the editor in the left hand pane.

This feature currently exists in Tina CMS:

https://tina.io/docs/contextual-editing/react/

They implement it by adding a data-tina-field property to clickable DOM elements which is populated by a function. Perhaps Payload could do something similar?