feat(prompt): prompt_getinput() gets current input #34491

Problem:
Not easy to get user-input in prompt-buffer before the user submits the
input. Under the current system user/plugin needs to read the buffer
contents, figure out where the prompt is, then extract the text.

Solution:
- Add prompt_getinput().
- Extract prompt text extraction logic to a separate function

src/nvim/edit.c

@@ -1078,7 +1078,7 @@ check_pum:
       return 0;
     }
     if ((mod_mask & MOD_MASK_SHIFT) == 0 && bt_prompt(curbuf)) {
-      invoke_prompt_callback();
+      prompt_invoke_callback();
       if (!bt_prompt(curbuf)) {
         // buffer changed to a non-prompt buffer, get out of
         // Insert mode