Settings: Add infinite scroll options to reading settings

[lsl]

For classic view simple sites we want to:

• Show the infinite scroll option from Writing Settings under Reading Settings in wp-admin.
• This option should only be shown when the site is using a classic theme. (e.g. 2012)
• Update the doc? https://wordpress.com/support/infinite-scroll/
  • -> Should be addressed in https://github.com/Automattic/en.support-docs-content/issues/3054.

We currently show a link:

But we want to match the behavior of atomic:

Note this option is also available in Jetpack but simple sites don't have access to Jetpack settings code.

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] Site Settings & Tools: The issue is focused on modifying site settings in the wp-admin, specifically related to reading settings.
• [Feature] Infinite Scroll: The feature being requested is specifically about adding an infinite scroll option, which directly aligns with this label.
• [Feature] Classic Interface: Since the changes are aimed at classic view simple sites and are dependent on the classic theme, this label is relevant.