feat: add require no space to argument short version

Fix #3030.

This PR enables restricting the short option to the -oval syntax with the require_no_space argument setting, and changes require_equals to only affect the long option --option=val (BREAKING CHANGE). This will allow compliance with the POSIX and GNU conventions as mentioned in the issue by setting both require_no_space and require_equals to true:

In clap terms, these are saying we should only allow

--long

--long=value

-s

-svalue

Here is an example help message that demonstrates this:

Usage: myapp [OPTIONS]

Options:
  -f[<foo>], --foo[=<foo>]  This is foo
  -h, --help                Print help

A specific real-world application that has motivated this is uutils/coreutils which aims to mirror the GNU version as closely as possible. However, their date -I/--iso-8601 implementation, for example, accepts values that are not considered valid (e.g. date -I seconds and date -I=seconds), and there is no possible workaround.

The changes made to require_equals may break current applications that take a positional argument after an optional option-argument, e.g.

Command::new("prog")
    .arg(
        Arg::new("cfg")
            .action(ArgAction::Set)
            .require_equals(true)
            .num_args(0..)
            .short('c'),
    )
    .arg(Arg::new("cmd"))
    .try_get_matches_from(vec!["prog", "-c", "cmd"]);

Currently cmd would be treated as the second argument, but after the PR where require_equals no longer works on short option, it will become the associated value to the first.

Migration to the new major version (if this is merged) should complement require_equals with require_no_space and replace -o=val usage with -oval, otherwise the associated value would become =val as = is parsed as part of the value.

clap-rs / clap

feat: add require no space to argument short version #5489