Add rule for repository.directory field

[greatislander]

Description of change

This PR adds a rule which can be used to enforce the presence of a repository.directory field, as per:

• https://github.com/npm/rfcs/blob/latest/implemented/0010-monorepo-subdirectory-declaration.md.
• https://github.com/npm/cli/releases/tag/v6.8.0

Checklist

• Unit tests have been added
• Specific notes for documentation, if applicable

[tclindner]

Thank you so much! I really appreciate you helping making this project better 🥇

Here is a link to the docs. Please let me know if you would like me to make any changes.
https://github.com/tclindner/npm-package-json-lint/wiki/require-repository-directory

[greatislander]

Docs look great, many thanks for accepting my PR!