[markdown] When typing enter/return in a list, automatically add new list item

What does this MR do and why?

When typing enter/return in a markdown list, automatically add new list items.

Related to #23977 (closed)

Feature flag: markdown_continue_lists

Screenshots or screen recordings

How to set up and validate locally

1. Enable the markdown_continue_lists feature flag in the rails c

   Feature.enable(:markdown_continue_lists)

2. Edit a description or comment and add markdown lists like you normally would

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.