When an edit is applied to the diagram, we want to handle that edit and show the resulting static model in the diagram (textually).

Add two functions, `applyEdit` and `getCurrentModel`, and use them when applying edits and showing the static model in the diagram.

Described in the tech design: https://docs.google.com/document/d/1ScR8DgLMBpeyQvgSPQ5iLHXpkugekeAJ74sUIj2CjHs/edit?tab=t.0#bookmark=kix.57zdltjdkvp5 

Display an error when the edit fails validation/parsing (also add a placeholder or template to the edit editor to help folks do their first edit). The edit events we need to handle are SetModel, AddRelationship, and RemoveRelationship.