Add argument `args` to `check_function()`

[mgirlich]

When taking a function as an argument the number or the name of the arguments are usually also important (e.g. see https://github.com/r-lib/httr2/pull/279 where this is useful).

This uses format_arg() from the standalone-cli.R file. Not sure if you want to add the dependency on this file as well or prefer a workaround.