feat: allow :wincmd to accept a count (#19815)

Let :wincmd command accept a count like what its documentation suggests.
Previously it could only accept a range, which led to some ambiguity on
which attribute should be used when executing :wincmd using nvim_cmd.

Closes #19662.

Also fix a typo in a related Vim test:

vim-patch:9.0.0223: typo in diffmode test

Problem:    Typo in diffmode test.
Solution:   Fix the typo. (closes vim/vim#10932)
5fd6ab820b

src/nvim/ex_cmds.lua

@@ -3181,7 +3181,7 @@ module.cmds = {
   },
   {
     command='wincmd',
-    flags=bit.bor(NEEDARG, WORD1, RANGE, CMDWIN, LOCK_OK),
+    flags=bit.bor(NEEDARG, WORD1, RANGE, COUNT, CMDWIN, LOCK_OK),
     addr_type='ADDR_OTHER',
     func='ex_wincmd',
   },

src/nvim/testdir/test_diffmode.vim

@@ -137,7 +137,7 @@ func Common_vert_split()
 
   " Test diffoff
   diffoff!
-  1wincmd 2
+  1wincmd w
   let &diff = 1
   let &fdm = diff_fdm
   let &fdc = diff_fdc