Add user preference to turn off automatically adding a new list item

What does this MR do and why?

[markdown] When typing enter/return in a list, ... (!79161 - merged) added the ability to automatically create new list items when pressing return. However many people prefer not to have the editor make that decision for them. This allows them to turn it off.

Screenshots or screen recordings

From the preferences, http://ee.gitlab.test:5100/-/profile/preferences, you can see the following in the Behavior section

How to set up and validate locally

1. go to a comment, start a list, like - one
2. press enter, you should see a new list item created
3. go to the profile preferences, http://ee.gitlab.test:5100/-/profile/preferences, and turn off the Automatically add new list items.
4. refresh the page with the comment
5. try adding a new list, pressing enter will not create a new list item

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.

Related to #363052 (closed)