[markdown] Typing `- - -` followed by Enter incorrectly adds a new list item

What does this MR do and why?

Valid horizontal rules can look like a list item, such as

- — -

We should not create a new list item for these when typing Enter

Instead of

- - -
- 

it should be

- - -

Same fix for * list items

Related to #353774 (closed)

How to set up and validate locally

From the rails c, enable the feature: Feature.enable(:markdown_continue_lists)

Try typing a valid markdown horizontal line, - -- and hit Enter. A new list item should not be created.

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