Add support for project_urls, similar to PyPI

I'd like to add an arbitrary link to our package's page on npmjs.com that is highlighted in the right-hand side navigation by adding it to our package's package.json file without adding it to ourREADME.md. Arbitrary URLs could include documentation, sponsorship, and other useful links. [Currently it appears that only homepageandrepository` are supported](https://docs.npmjs.com/creating-a-package-json-file#default-values-extracted-from-the-current-directory).

PyPI offers this feature in either its setup.py or pyproject.toml project file.

[project.urls]
Repository = "https://github.com/plone/plone-sphinx-theme"
Documentation = "https://plone-sphinx-theme.readthedocs.io/"

This renders as shown in the screenshot and following URL.

https://pypi.org/project/plone-sphinx-theme/

Is this something you would consider useful?

Review the package.json metadata, especially the existing homepage and repository support, and compare it with the linked PyPI project.urls example. Determine where arbitrary links should be documented and what rendering or behavior would define completion; the issue does not identify an implementation file or test.