Unison local Definition Diffs API

[ChrisPenner]

Overview

Adds definition diff API to Unison Local.

Implementation notes

Adds /api/projects/<slug>/diff/terms and /api/projects/<slug>/diff/types which return the same format as Share

Test coverage

Added a simple transcript test

Loose ends

Not sure when we'll have time to add this to the UI, but at the very least it serves as a mechanism to add some testing for the feature in unison rather than just in share-api cc @hojberg

[aryairani]

@ChrisPenner go ahead and merge these once the tests are passing