Improve Prismic model differ output

[davidpmccormick]

Post Slice Machine migration update to ticket

See #10860 where we removed the diffing task

However, we still want a similar script to be used as a local tool. The Slice Machine tool's interface tells us something has changed, but not exactly what. This tool would serve to bridge that gap.

• [ ] If needed, ensure README documents are updated as part of this work

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Original ticket

Updates to the model diffing in #7605 can lead to potentially misleading output (the screenshot below might make it look a bit like the Event type doesn't currently exist).

It would be good to find a way to indicate this isn't the case/give a bit more context so we can have confidence we're changing the right thing.

[rcantin-w]

@davidpmccormick can this ticket be closed due to Slice Machine work?

[rcantin-w]

To be reviewed post slice machine work