Add MIT-0 (MIT No Attribution) license support

## Summary

MIT-0 (MIT No Attribution) is an OSI-approved license (approved August 14, 2020) with SPDX identifier `MIT-0`. It is a variant of the MIT license that removes the attribution requirement.

## Problem

Running `identify_license` on an MIT-0 licensed file returns no license match. The license body is not recognized, so tools built on top of licenseclassifier (such as [google/go-licenses](https://github.com/google/go-licenses)) classify MIT-0 as `Unknown`.

## Proposed Fix

Add MIT-0 license text as a new asset under `v2/assets/License/MIT-0/`.

The canonical MIT-0 text (from https://opensource.org/license/mit-0):

```
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
```

## References

- SPDX: https://spdx.org/licenses/MIT-0.html
- OSI approval: https://opensource.org/license/mit-0
- AWS repository: https://github.com/aws/mit-0