chore: add license to package.json

[smackfu]

Closes #461

This PR adds the license key to the package.json https://docs.npmjs.com/cli/v9/configuring-npm/package-json#license

The "Unlicense" value is from the SPDX license list: https://spdx.org/licenses/

Adding the license to the package.json improves support for automated license checkers that only have access to the package.json.