fix(ui): avoid redundant ext_cmdline events (#32237)

Problem:  `cmdline_show` is emitted unnecessarily each event
          loop iteration, because `cmdline_was_last_drawn` is never set.
Solution: Keep track of whether the cmdline was last drawn to avoid
          unnecessarily emitting cmdline_show. Set `redraw_state` to
          emit `cmdline_pos` when emitting `CursorMovedC`. Only emit
          `cmdline_pos` when cmdline was last drawn.

test/functional/lua/ui_event_spec.lua

@@ -202,7 +202,7 @@ describe('vim.ui_attach', function()
     feed([[:call confirm("Save changes?", "&Yes\n&No\n&Cancel")<CR>]])
     screen:expect({
       grid = [[
-        ^5                                       |
+        ^4                                       |
         {1:~                                       }|*4
       ]],
       cmdline = {
@@ -224,7 +224,7 @@ describe('vim.ui_attach', function()
     feed('n')
     screen:expect({
       grid = [[
-        ^5                                       |
+        ^4                                       |
         {1:~                                       }|*4
       ]],
       cmdline = { { abort = false } },