Let Content Model handle ENTER key

[JiuqingSong]

This covers the following scenarios:

1. Enter on normal paragraph, split the paragraph and bring focus to the new line
2. Enter on list item, split the list item
3. Enter on empty list item, reduce list level
4. Enter on empty quote line, unquote this line
5. Enter on expanded selection, first delete the selected content, then do the action above
6. Shift+Enter, always split current paragraph

[JiuqingSong]

#2586 #2510