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/optionstr.c

@@ -28,6 +28,7 @@
 #include "nvim/indent_c.h"
 #include "nvim/insexpand.h"
 #include "nvim/macros_defs.h"
+#include "nvim/mark.h"
 #include "nvim/mbyte.h"
 #include "nvim/memline.h"
 #include "nvim/memory.h"
@@ -695,6 +696,11 @@ const char *did_set_buftype(optset_T *args)
       || opt_strings_flags(buf->b_p_bt, opt_bt_values, NULL, false) != OK) {
     return e_invarg;
   }
+  // buftype=prompt: set the prompt start position to lastline.
+  if (buf->b_p_bt[0] == 'p') {
+    pos_T next_prompt = { .lnum = buf->b_ml.ml_line_count, .col = 1, .coladd = 0 };
+    RESET_FMARK(&buf->b_prompt_start, next_prompt, 0, ((fmarkv_T)INIT_FMARKV));
+  }
   if (win->w_status_height || global_stl_height()) {
     win->w_redr_status = true;
     redraw_later(win, UPD_VALID);