🚀 Feature: Create valid-module

[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

Following #42 -> #51: we've resolved to create a standalone valid-man rule. It should enforce that, if the field exists in the package.json file, it's set to a valid value. It should do nothing if the field doesn't exist. Per #850, this should be included in the recommended config.

Additional Info

For this issue, we should:

1. Send a PR to create the rule in this repo (resolving this issue)
2. File an issue on https://github.com/JoshuaKGoldberg/package-json-validator to create a standalone API that performs the same validation
3. Once a version of package-json-validator is released with that feature, file an issue on this repo to switch the rule to use that API from package-json-validator

If you want to take on just step 1, that's ok - no pressure to do all three. 🙂

Once all valid- rules exist in this plugin, we can then file an issue to deprecate the valid-package-def rule in favor of them.