License field should contain the license identifier, not the path to the license file

[musicinmybrain]

Instead of "./LICENSE.txt", the license field in package.json should contain a standard SPDX license identifier. It looks like the intended license is "BSD-3-Clause".

Currently, https://www.npmjs.com/package/thesaurus shows “License: none” because of this, and users are unable to automatically determine the license when thesaurus is used as a dependency, which may prevent some from adopting it.