fix: Added support for the UNLICENSED NPM magic value (fixes #113)

[mat-sz]

As far as I'm aware only all caps spelling (with alternative UNLICENCED spelling) is allowed (see: validate-npm-package-license.js).

This resolves that issue, the library correctly treats UNLICENSED as non-green (all copyright retained). I've added a test case for that as well.

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[mat-sz]

@googlebot I signed it!

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[JustinBeckwith]

Awesome - thank you!