Feature Request / Feature Parity: Option to require users to be logged into a WordPress.com account to comment

### What

A new option to require `Users must be logged in to WordPress.com to comment`.

Related internal discussion (note that some unrelated items where also discussed in the thread and can be ignored): p9F6qB-7Ma-p2#comment-43996

### Why

On WordPress.com Simple, the core WordPress option for `Users must be registered and logged in to comment` behaves differently than it does on Atomic, Jetpack, and self-hosted sites.

On Simple, `Users must be registered and logged in to comment` allows site owners to require users be logged into WordPress.com to comment. This doesn't require the user to be registered on the site itself, just WordPress.com.

Per the [WordPress.com documentation](https://wordpress.com/support/settings/discussion-settings/#other-comment-settings) for this option:

> Users must be registered and logged in to comment – If this box is checked, only logged in WordPress.com users will be allowed to leave comments. If it is not checked, any visitor can leave a comment.

#### Atomic & Self-hosted

When `Users must be registered and logged in to comment` is enabled on Atomic, it follows self-hosted behavior and requires that the user be registered on the site itself.

This means that simply being logged into a WordPress.com account will not suffice and that account must be a registered user on the site.

The option is also [incompatible with the social login options provided by Jetpack](https://jetpack.com/support/comments/)'s `Let visitors use a WordPress.com, Twitter, Facebook, or Google account to comment.`

Related internal discussion (note that some unrelated items where also discussed in the thread and can be ignored): p9F6qB-7Ma-p2#comment-43996

### How

Explore providing users with a new option that doesn't require users to be registered on the site but allows them to require commenters to be logged into a WordPress.com account.

Start with the linked internal discussion and the WordPress.com discussion-settings documentation, then compare the existing comment-login behavior on Simple, Atomic, Jetpack, and self-hosted sites. The issue is complete when the proposed option's scope and behavior are defined for commenters who are logged into WordPress.com but are not registered on the site.

Written by the indexing model from the issue text.