Added support for multiple licenses

NPM allows for multiple licenses in the form of SPDX expressions. For example, "(ISC OR GPL-3.0)".

Currently, the NpmMapping().normalize_license method does not cover such cases. I have added support for this, and it returns a list of licenses if there are multiple licenses.

It passes the tests (Ran the tests using Pytest). However, the current tests do not cover multiple licenses.

Closes #4480 (closed)