feat(prompt): multiline prompt input #33371

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.

src/nvim/buffer_defs.h

@@ -699,6 +699,7 @@ struct file_buffer {
   Callback b_prompt_interrupt;  // set by prompt_setinterrupt()
   int b_prompt_insert;          // value for restart_edit when entering
                                 // a prompt buffer window.
+  fmark_T b_prompt_start;       // Start of the editable area of a prompt buffer.
 
   synblock_T b_s;               // Info related to syntax highlighting.  w_s
                                 // normally points to this, but some windows