Autoformat --- for horizontal rule

[zadam]

📝 Provide a description of the new feature

Typing --- should insert horizontal rule just like in Markdown (btw this is used below in this issue template). This was mentioned in the #1365 but marked as "not a part of MVP".

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[neongreen]

Looks like this is pretty easy to add:

https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-autoformat/src/autoformat.js#L165-L167

[Reinmar]

The tricky thing is that autoformat is about formatting and here we want to insert something. We'd need to check whether it'll actually work. Unless, you checked it already?

[neongreen]

Haven't checked, unfortunately. It didn't occur to me there could be a difference between formatting and insertion.

[maxbarnas]

A few interesting cases regarding this autoformatting feature:

Non-intrusive

• ---^foobar creates new paragraph with foobar below the newly created horizontal line.
• Blockquote allows the horizontal line to be created inside itself.
• --- inside empty headings leaves the heading, then creates the horizontal line, then a new paragraph.
• ---^heading leaves one heading, creates a horizontal line, and creates a new heading with the original content, heading.

Slightly confusing

• --- inside the list (numbered or not) creates a horizontal line below the list item.
• There is a clash with automatic text transformation.

[maxbarnas]

Typical behavior in other editors:

• CKE5
  • -- => – (en-dash).
  • --- => — (em-dash).
• CKE4
  • none? 
• Slack post editor and Notion:
  • -- => — (em-dash).
  • --- => horizontal line.
• LibreOffice Writer
  • -- => — (em-dash).
  • ---<Enter> (or more -) => horizontal line (check Apply Borders in AutoCorrect options dialog).
• Word 365
  • -- => – (en-dash).
  • ---<Enter> => horizontal line (only inside empty paragraphs).
• Google Docs
  • Autoformat is disabled by default. When turned on, there's no default rules for dashes whatsoever.