fix: use a valid SPDX license identifier in package.json

fix: use a valid SPDX license identifier in package.json

As per:

https://docs.npmjs.com/files/package.json#license

Note that even though I called this "public domain", the actual text in
the LICENSE.md is in fact "The Unlicense":

https://spdx.org/licenses/Unlicense.html

which is pretty much the closest thing that we have to a standard way to
dedicate something to the public domain.