Feature/Bug: multiline headings & markdown

[benclarkmck]

Description

When pressing shift+enter while in heading mode, a new line is created and heading is still selected, however when you go to markdown view the generated markdown has the second line without a heading and this will then convert back to normal text when viewing the rendered dom view

Screen.Recording.2024-09-24.at.09.41.25.mov

I have 2 alternate solutions:

Option 1

Code: https://github.com/facebook/lexical/compare/main...benclarkmck:lexical:multiline-heading-option-1?expand=1

Convert this to # Line 1\n# Line 2 leaving the export as-is. This will only change the behaviour when going from dom to markdown and create no change in behaviour if lexical is used to render this markdown - it will be imported to two heading tags currently but then going back to markdown this will then export back to having a paragraph break \n\n between the headings

Screen.Recording.2024-09-24.at.09.43.30.mov

Option 2

Code: https://github.com/facebook/lexical/compare/main...benclarkmck:lexical:multiline-heading-option-2?expand=1

Convert this as in option 1 to #Line 1\n#Line 2 but keep the line break <br> within the 1 heading tag when importing. This keeps consistency between the markdown and the dom view, but is a change in behaviour if lexical is used to render any markdown with headings separated by a single line break

Screen.Recording.2024-09-24.at.09.42.31.mov

Third option (not implemented)

Another option is to not support line breaks in headings and instead go straight back to normal text, or create a new heading tag in the dom rather than line break when shift+enter, INSERT_LINE_BREAK_COMMAND command, is used while in heading mode

Impact

The current bug caused some confusion when user created shift+enter while typing a heading, so any solution would fix it.