feat(ui)!: emit prompt "messages" as cmdline events #31525

Problem:  Prompts are emitted as messages events, where cmdline events
          are more appropriate. The user input is also emitted as
          message events in fast context, so cannot be displayed with
          vim.ui_attach().
Solution: Prompt for user input through cmdline prompts.

test/functional/vimscript/null_spec.lua

@@ -116,7 +116,7 @@ describe('NULL', function()
     null_expr_test(
       'is accepted as an empty list by inputlist()',
       '[feedkeys("\\n"), inputlist(L)]',
-      'Type number and <Enter> or click with the mouse (q or empty cancels): ',
+      '',
       { 0, 0 }
     )
     null_expr_test(