Cache outline views per file

[joshspicer]

When a user has the follow mode (📌 ) on navigating back and forth between 2 files will repeatly cause the diagrams to query the model to generate a new diagram. If the target file hasn't changed a lot, we're likely wasting the user's time by not just showing the previously generated diagram.

We should cache file outlines and show those unless the file has changed ("changed" could be any change, or perhaps a substantial change?) to make things feel snappier and reduce interaction with the model.

We could borrow our quick iteration tactics with /iterate to make incremental updates to a diagram.