Verify that all asset licenses are accounted for in `CREDITS.md`

## What problem does this solve or what need does it fill?

As discussed in https://github.com/bevyengine/bevy/pull/4975#discussion_r894906706, we should verify that all assets are correctly attributed.

## What solution would you like?

1. Add a `LICENSES.csv` file to the `assets` folder.
2. Record all licenses and source links there.
3. Add a step to CI to verify that there's a one-to-one correspondence.

## What alternative(s) have you considered?

Manually verify that these are correct.
This solves the problem for now, but doesn't increase our long-term confidence.