feat(ex_cmds): :sleep! hides the cursor while sleeping (#31493)

Problem:  :sleep! not hiding the cursor is an arbitrary difference from
          Vim without obvious justification, and Vim's behavior isn't
          easily achievable in Nvim.
Solution: Make :sleep! hide the cursor while sleeping.

Ref:
6a01b3fcc3
b5c0ade437

test/functional/ui/cursor_spec.lua

@@ -361,4 +361,38 @@ describe('ui/cursor', function()
       end
     end)
   end)
+
+  it(':sleep does not hide cursor when sleeping', function()
+    n.feed(':sleep 100m | echo 42\n')
+    screen:expect({
+      grid = [[
+      ^                         |
+      {1:~                        }|*3
+      :sleep 100m | echo 42    |
+    ]],
+      timeout = 100,
+    })
+    screen:expect([[
+      ^                         |
+      {1:~                        }|*3
+      42                       |
+    ]])
+  end)
+
+  it(':sleep! hides cursor when sleeping', function()
+    n.feed(':sleep! 100m | echo 42\n')
+    screen:expect({
+      grid = [[
+                               |
+      {1:~                        }|*3
+      :sleep! 100m | echo 42   |
+    ]],
+      timeout = 100,
+    })
+    screen:expect([[
+      ^                         |
+      {1:~                        }|*3
+      42                       |
+    ]])
+  end)
 end)