fix: use html entity for consecutive newlines (fix #2394 #2586)

[kshnurov]

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Description

Consecutive newlines could be made by using html entities, e.g. space  , which are completely compliant with the markdown spec. Current implementation with <br> breaks a lot of things.

This PR replaces <br> with a space HTML-entity  , and also reverts #2598 for the following reasons:

1. You instantly get an invalid markdown if you add an empty line to a list item
2. Having 2 separate lists with only a newline between them is a rare and weird case, while having a newline in a list item is much more common.

I understand why you want markdown to reflect WYSIWYG as much as possible, but have you ever had a case when you needed 2 lists separated by an empty line? I can't imagine one, so let's consider it a feature - auto-merging of consecutive lists :)

[kshnurov]

@jajugoguma @jwlee1108 what do you think about this one and a new version?

[stale]

This issue has been automatically marked as inactive because there hasn’t been much going on it lately. It is going to be closed after 7 days. Thanks!

[kshnurov]

@jajugoguma @jwlee1108