Problem:
Cannot enter multiline prompts in a buftype=prompt buffer.
Solution:
- Support shift+enter (`<s-enter>`) to start a new line in the prompt.
- Pasting multiline text via OS paste, clipboard, "xp, etc.
- A/I in editable region works as usual.
- i/a/A/I outside of editable region moves cursor to end of current
prompt.
- Support undo/redo in prompt buffer.
- Support o/O in prompt buffer.
- Expose prompt location as `':` mark.
286371b4d2
