Add support for short subtag in core:flags

[prnvbn]

I want to add suport for long and short unix flags (for e.g. -f/--flags) when using core:flags. for e.g.

Lets take the following contrived example

package sample

import "core:flags"
import "core:fmt"
import "core:os"

Options :: struct {
	version: bool `usage:"show version number"`,
	echo:    string `usage:"echo arg"`,
}

main :: proc() {
	opts: Options
	flags.parse_or_exit(&opts, os.args, .Unix)

	switch {
	case opts.version:
		fmt.println("dev")
	case:
		fmt.println(opts.echo)
	}
}

I can run it like so

❯ odin run sample.odin  -file -- --echo hello
hello
❯ odin run sample.odin  -file -- version
dev

however, if I want to support shorthand flags, it is quite cumbersome to set it up (especially if you want the usage to be nice)

I would like support for a "short=..." subtag under the args tag so, that I can define my Options struct as follows:

Options :: struct {
	version: bool `args:"short=v" usage:"show version number"`,
	echo:    string `args:"short=e" usage:"echo arg"`,
}
// odin run sample.odin  -file -- --echo hello
// odin run sample.odin  -file -- -echo hello
// odin run sample.odin  -file -- -e hello
// should all result in the output hello

[prnvbn]

created this PR - #7001

P.S. I couldn't find a CONTRIBUTING.md and the discord link in the README didn't work for me so apologies if I have missed something