Add support for joined short options

[natecook1000]

Description

This supports joined arguments, like '-Ddebug' or '-v4'. Joined arguments need to be explicitly declared as .customShort("D", allowingJoined: true).

Detailed Design

You can now specify a Boolean allowingJoined value when declaring a custom short name specification, like this:

@main
struct Example: ParsableCommand {
    @Option(name: .customShort("D", allowingJoined: true))
    var debugValue: String

    func run() {
        print(debugValue)
    }
}

Existing calls still work as expected:

$ example -D debug
debug
$ example -D=debug
debug

And now you can use joined syntax:

$ example -Ddebug
debug

Documentation Plan

Expanded the documentation for .customShort(_:allowingJoined:) on both NameSpecification and NameSpecification.Element.

Test Plan

Unit tests covering the breadth of usage.

Source Impact

This is an additive change only, users must opt into the new functionality.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[mpapple-swift]

Reviewed code; tested on Darwin. Looks good. Thanks for adding this enhancement @natecook1000.

[natecook1000]

@swift-ci Please test