🚀 Feature: Use npm modules for validation

[jasonkarns]

The version field probably ought to be validated using the semver module, and the license field probably ought to be validated using the spdx module. Both of these are used by normalize-package-data which in turn is used by npm itself when parsing the package.json.

Other parsing/validation rules (for the npm spec, at least) ought to be brought in line with the normalization rules from normalize-package-data

[JoshuaKGoldberg]

This would be great: the closer this package gets to the npm spec, the better!

Per #78 I'm still ramping up on this project, so I don't want to take on any big changes anytime soon. Marking this one as status: blocked until the existing slate of smaller fixes & feature request come through.