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/eval.lua

@@ -8332,6 +8332,21 @@ M.funcs = {
     signature = 'printf({fmt}, {expr1} ...)',
     returns = 'string',
   },
+  prompt_getinput = {
+    args = 1,
+    base = 1,
+    desc = [=[
+      Gets the current user-input in |prompt-buffer| {buf} without invoking
+      prompt_callback. {buf} can be a buffer name or number.
+
+      If the buffer doesn't exist or isn't a prompt buffer, an empty
+      string is returned.
+
+    ]=],
+    name = 'prompt_getinput',
+    params = { { 'buf', 'integer|string' } },
+    signature = 'prompt_getinput({buf})',
+  },
   prompt_getprompt = {
     args = 1,
     base = 1,