vim-patch:8.2.3400: ":z!" is not supported

Problem:    ":z!" is not supported.
Solution:   Make ":z!" work and add tests. (Dominique Pellé, closes vim/vim#8836)
            Use display height instead of current window height.
7f2dd1e90c

src/nvim/ex_cmds.lua

@@ -3271,7 +3271,7 @@ module.cmds = {
   },
   {
     command='z',
-    flags=bit.bor(RANGE, WHOLEFOLD, EXTRA, FLAGS, TRLBAR, CMDWIN),
+    flags=bit.bor(RANGE, WHOLEFOLD, BANG, EXTRA, FLAGS, TRLBAR, CMDWIN),
     addr_type='ADDR_LINES',
     func='ex_z',
   },