Problem:
nvim_cmd() duplicates Ex parsing: it sets the address type and default
range itself and builds the whole command line in build_cmdline_str().

Solution:
Build only the range, command name and arguments, and let
parse_cmdline() fill in exarg_T.

Just Say No to indirection.

Problem:
`magic` was owned by `CmdParseInfo`, but command handlers
need it at execution time via `exarg_T`.

Solution:
Move `magic` into `exarg_T`, and initialize it in `parse_cmdline()`.

Problem:
No lint for strtol().

Solution:
Add lint, and update existing usages.

Callers that previously got a (garbage) large positive value:
- `indent.c`: use getdigits() (intmax, no int truncation) with def=1 so
  overflow/too-large values stay positive and fall through to the
  existing "too big" check (E475).
- `file_search.c`: use getdigits() with def=255 so overflow/too-large
  values keep the "max expand" (else) branch.

The other conversions (api/command.c, eval/window.c, highlight_group.c,
tui/tui.c) only diverge on pathological overflow inputs where strtol's
result was already garbage and the observable outcome is unchanged.

Problem:  User commands cannot handle single args with spaces
Solution: Add the -nargs=_ attribute (Maxim Kim)

-nargs=_ allow user commands to have a single argument with spaces.

For example given the following Test command and TestComplete function:

```
vim9script
def TestComplete(A: string, _: string, _: number): list<string>
    var all = ["qqqq", "aaaa", "qq aa"]
    return all->matchfuzzy(A)
enddef
command! -nargs=_ -complete=customlist,TestComplete Test echo <q-args>
```

`:Test q a<tab>` should successfully complete `qq aa`

fixes:  vim/vim#20102
closes: vim/vim#20189

https://github.com/vim/vim/commit/f0e874a129a702457a383017b576eef1553f95d8

Co-authored-by: Maxim Kim <habamax@gmail.com>

Problem:
Linter missed backtick and double-quote keynames in the quasi-keyset of
the `nvim_create_user_command` docstring.

Solution:
Update the linter to check backtick-surrounded and quote-surrounded key
names.

Problem:
`excmd_get_argt` and `get_cmd_argt` do the same thing.

Solution:
Drop `get_cmd_argt` and update its callers to use `excmd_get_argt`.