🚀 Feature: export validateAuthor function

[michaelfaith]

Feature Request Checklist

• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

In support of JoshuaKGoldberg/eslint-plugin-package-json#840, package-json-validator should provide a validateAuthor function, which validates the value of the author property in a package.json. The function should validate whether the value is of the correct form. As part of introducing the new function, it should be used in place of the equivalent validation for author in the monolithic validate function.

Additional Info

The current monolithic validate function uses a validatePeople function that does a pretty good job of validating the shape of author. So, validateAuthor should just be a light wrapper around this function.