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

src/nvim/ex_docmd.c

@@ -6101,7 +6101,9 @@ static void ex_sleep(exarg_T *eap)
   default:
     semsg(_(e_invarg2), eap->arg); return;
   }
-  do_sleep(len, false);
+
+  // Hide the cursor if invoked with !
+  do_sleep(len, eap->forceit);
 }
 
 /// Sleep for "msec" milliseconds, but return early on CTRL-C.