Support "SPDX Short Identifier" expressions

## Short Description

"SPDX Short Identifier" is a thing and we may not always detect these.
See https://github.com/search?q="SPDX+short+identifier"&type=code

## Possible Labels

- new feature

## Select Category

- [x] Enhancement
- [ ] Add License/Copyright
- [x] Scan Feature
- [ ] Packaging
- [ ] Documentation
- [ ] Expand Support
- [ ] Other