Improve markdown newline export/import

[fantactuka]

Adjustments around how markdown import/export deal with new lines:

• Export will use \n\n as block separator
• Import ignores empty paragraphs (but keeps empty quotes, lists, headings)
• Paragraph content is appended to the previous sibling if it is not empty separated by hard line breaks
• Multi-line blockquote content is appended

These changes should make our markdown a bit closer to commonmark spec. It still has some differences, e.g. we do not support nested quotes (in general we do not nest quotes in editor) or soft line breaks

input:

Paragraph text
continuation

Another paragraph
- List item
continuation
> Quote line 1
> line 2
continuation

output:

<p>
  Paragraph text<br>
  continuation
</p>
<p>
  Another paragraph
</p>
<ul>
  <li>
    List item<br>
    continuation
  </li>
</ul>
<bloquoute>
  Quote line 1<br>
  line 2<br>
  continuation
</bloquoute>

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
lexical	✅ Ready (Inspect)	Visit Preview	Jun 27, 2022 at 11:49AM (UTC)
lexical-playground	✅ Ready (Inspect)	Visit Preview	Jun 27, 2022 at 11:49AM (UTC)

[jearthman]

@fantactuka @zurfyx

I think this commit is causing an issue with whitespace being parsed out at the end of the root node's text when using markdown convert functions for two-way data binding. Explanation of my issue here with code snippet.

Specifically this line: const lineTextTrimmed = lineText.trim()

https://discord.com/channels/953974421008293909/1193984435406778388/1193984435406778388