Resolve "With :markdown_continue_lists ANY "Enter" adds new list item"

What does this MR do and why?

When using autocomplete in a markdown list item, using "Enter" to select an item caused a new markdown list item to be created.

We now check atwho('isSelecting') to determine if the keydown event was generated by atWho, the autocomplete engine, and don't do our own markdown list continuation processing.

Screenshots or screen recordings

How to set up and validate locally

Make sure the feature flag markdown_continue_lists is enabled

MR acceptance checklist

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Related to #353245 (closed)