Follow-up from "Auto-increment markdown lists": refactor certain methods/variable for clarity

The following discussion from !81079 (merged) should be addressed:

• @mrincon started a discussion:

  Suggestion (non-blocking): This was not introduced here, but the terminology around these concepts is inconsistent, all of these refer to similar ideas:

  lineBefore currentLine, prevLineEmpty

  I had trouble understanding this code because of this, I would love it if we can follow this one up with a refactor to consolidate terms.