Show diff between latest and staging Asset Trees when executing "add"

[bafu]

Currently, Nit shows the latest and the staging Asset Trees separately.

Showing the diff will increase the UX.

[bafu]

Candidates

1. https://github.com/andreyvit/json-diff, 3-day ago, 777 stars
2. https://github.com/kpdecker/jsdiff, 29-day ago, 6,383 stars
3. https://github.com/benjamine/jsondiffpatch, 13-month ago, 4,101 stars
4. https://github.com/ltwlf/json-diff-ts, 2-month ago, 21 stars