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#20102closes: vim/vim#20189f0e874a129
Co-authored-by: Maxim Kim <habamax@gmail.com>
7b46848f14
