🚀 Feature: Bring in a valid-homepage 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-homepage 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. The valid-package-def rule already contains this logic using package-json-validator. For this issue, we should:

1. File an issue on https://github.com/JoshuaKGoldberg/package-json-validator to create a standalone API to validate this specific field
2. Send a PR to add API to package-json-validator
3. Once a version of package-json-validator is released with this field's API, mark this issue as unblocked
4. Send a PR to this repo adding in a standalone rule for this field - without adding it to the recommended config

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.

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