Support line breaks

[stevengharris]

Sometimes it's just very useful to insert a line break within a styled element. This is supported in Markdown. A "normal" editor practice would be to hot-key to Shift+Enter. This will require corresponding support for Undo and Redo and perhaps some way to enable for touch/keyboardless devices. See also the discussion in https://github.com/stevengharris/MarkupEditor/issues/128#issuecomment-1703950127, where newlines come in via paste. Without a corresponding ability to insert newlines or line breaks during editing, text you paste in can look different than anything you can produce yourself, which is inconsistent.