Identify asset usage

## Describe the task

Removing unused assets or re-organising assets that are only used for tests or sketches etc. is not simple or intuitive to do in Unity and so often is just not done. There are some existing tools that do some of this but seem either inflexible enough to still be time consuming to use or error prone.

## Completed When

- [ ] identify assets (exclusive) usage
- [ ] by folder (so we can move or remove assets that the game won't use)
- [ ] identify assets that are in a build

## Additional Context
perhaps a grid layout or tick boxes to so you can show or exclude build referenced

Begin by surveying the existing Unity asset-usage tools and how build references are represented; the issue names no files, tests, or entry points. Done means the tool identifies exclusively used assets by folder, identifies assets included in a build, and provides a way to show or exclude build-referenced assets.

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