This matches the code in execute_common(), preventing messages after the
API call from being printed at the wrong column.

This avoids including ex_eval.h in any other header, thus preventing
future circular includes.

vim-patch:8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
https://github.com/vim/vim/commit/a80faa8930ed5a554beeb2727762538873135e83

Closes #19696

Problem:    Support for user commands is spread out. No good reason to make
            user commands optional.
Solution:   Move user command support to usercmd.c.  Always enable the
            user_commands feature.
https://github.com/vim/vim/commit/ac9fb18020d7e8bf16d02d45fbb02cf47328aaf7

Co-authored-by: ii14 <ii14@users.noreply.github.com>

The last commit didn't actually disable V1028 because of a typo.
Fix the typo so it is actually disabled.

Reduce the amount of string allocations and length calculations.

With the following benchmark:

```lua
total = 0
for _ = 1, loops do
  local start = now()
  vim.api.nvim_cmd({cmd = 'let', args = {'a', '=', '1'}}, {})
  total = total + (now() - start)
end
print('nvim_cmd', total / loops)
```

```
hyperfine 'nvim --clean test.lua +source +q'
```

Before: 234.5ms
After:  173.8ms