fix(api): update grid cursor in nvim_win_set_cursor()

Problem:  Cursor position set by nvim_win_set_cursor() is not reflected
          on the screen when followed by a blocking call like getchar().
Solution: Immediately update the cursor position on the grid.

src/nvim/api/window.c

@@ -148,7 +148,7 @@ void nvim_win_set_cursor(Window window, ArrayOf(Integer, 2) pos, Error *err)
   switchwin_T switchwin;
   switch_win(&switchwin, win, NULL, true);
   update_topline(curwin);
-  validate_cursor(curwin);
+  setcursor_mayforce(true);
   restore_win(&switchwin, true);
 
   redraw_later(win, UPD_VALID);

test/functional/api/window_spec.lua

@@ -219,6 +219,21 @@ describe('API/win', function()
 
       -- curwin didn't change back
       neq(win, curwin())
+
+      -- shows updated position after getchar() #20793
+      feed(':call getchar()<CR>')
+      api.nvim_win_set_cursor(win, { 1, 5 })
+      screen:expect {
+        grid = [[
+                                      |
+        {1:~                             }|*2
+        {2:[No Name]                     }|
+        prolo^gue                      |
+                                      |*2
+        {3:[No Name] [+]                 }|
+        :call getchar()               |
+      ]],
+      }
     end)
 
     it('remembers what column it wants to be in', function()