feat: allow :wincmd to accept a count (neovim#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 neovim#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)
vim/vim@5fd6ab8

runtime/doc/vim_diff.txt

@@ -372,6 +372,7 @@ Lua interface (|lua.txt|):
 
 Commands:
   |:doautocmd| does not warn about "No matching autocommands".
+  |:wincmd| accepts a count.
 
 Functions:
   |input()| and |inputdialog()| support for each other’s features (return on

runtime/doc/windows.txt

@@ -442,6 +442,7 @@ position is set to keep the same Visual area selected.
 These commands can also be executed with ":wincmd":
 
 :[count]winc[md] {arg}
+:winc[md] [count] {arg}
 		Like executing CTRL-W [count] {arg}.  Example: >
 			:wincmd j
 <		Moves to the window below the current one.

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

test/functional/ex_cmds/wincmd_spec.lua

@@ -0,0 +1,13 @@
+local helpers = require("test.functional.helpers")(after_each)
+local clear = helpers.clear
+local eq = helpers.eq
+local funcs = helpers.funcs
+local command = helpers.command
+
+it(':wincmd accepts a count', function()
+  clear()
+  command('vsplit')
+  eq(1, funcs.winnr())
+  command('wincmd 2 w')
+  eq(2, funcs.winnr())
+end)