🚀 Feature: Bring in a valid-keywords rule

[JoshuaKGoldberg]

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• 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-keywords 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.

Additional Info

Per discussion in #51, this won't be enabled by default at first. 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.

💖

[JoshuaKGoldberg]

Note: michaelfaith and I talked over the strategy. I'd initially put that the validation API should exist in package-json-validator first, then be ported here. Michael suggested letting folks make it here first for an easier contribution process and faster determining whether how the API should look the rule. I agree, that's much better 😄. Edited.