Add support for SPDX license expression syntax

[Simek]

NPM users can set the license field using SPDX license expression syntax:

• https://docs.npmjs.com/cli/v7/configuring-npm/package-json#license

Currently those expressions are resolved as Unrecognized license, it would be nice to support this too. The recommended package to parse the syntax is:

• https://www.npmjs.com/package/spdx-expression-parse