fix(messages): recognize cmdline one_key/number prompt State (#34206)

Problem:  Since 48e2a736, prompt messages are handled by an actual
          active cmdline, resulting in `State` no longer being equal
          to `MODE_CONFIRM` which is used in some places. E.g. to
          specify the current `mode()` or to re-emit a confirm message.
Solution: Replace `MODE_CONFIRM` with a new `MODE_CMDLINE` sub-mode when
          `ccline.one_key/mouse_used` is set. Use it to avoid clearing
          mouse_used prompt messages, and to re-emit one_key messages
          (when ext_messages is inactive, for which this is unnecessary).

test/functional/api/vim_spec.lua

@@ -2157,6 +2157,11 @@ describe('API', function()
       feed('<F2>')
       eq({ mode = 'n', blocking = false }, api.nvim_get_mode())
     end)
+
+    it('returns "c" during number prompt', function()
+      feed('ifoo<Esc>z=')
+      eq({ mode = 'c', blocking = false }, api.nvim_get_mode())
+    end)
   end)
 
   describe('RPC (K_EVENT)', function()