Add ConfigurationBuilder for custom command configurations

[cmcgee1024]

See issue #36 for an additional example.

[iCharlesHu]

Could you elaborate more on what's the benefit of this approach over declaring static vars on Configuration? For example:

extension Configuration {
    static var ls: Self {
        return Configuration(
            executable: .path("/bin/ls"),
            ...
        )
    }
}

The run() methods that take a Configuration was designed specifically for the use case above (I just realized I forgot to include a collected result version of the overload which I'll add).

let result = try await run(.ls)

[cmcgee1024]

let result = try await run(.ls)

I'm not seeing how this var can build a command-line with the arguments provided at the call-site. How do I provide a specific directory "foo" with this approach and specify the long option?

With an Ls struct that conforms to the CommandBuilder protocol then I can do it like this:

let result = try await run(
    Ls(.long, paths: "foo")
)