feat: add Args.option() for typed arg options

[rexxars]

Adds a new Args.option() method mirroring Flags.option(), which provides type-safe option usage:

class Deploy extends Command {
  static args = {
    env: Args.option({
      options: ['production', 'staging', 'development'] as const
    })()
  }

  async run() {
    const {args} = await this.parse(Deploy)
    // `args.env` typed as `'production' | 'staging' | 'development' | undefined`
  }
}

Setting it as required removes the undefined from the union.

[rexxars]

Updated to include stricter type parsing: I feel it is easy to assume that options like multiple: true would go alongside options, but they should be passed on the second call.

I think theoretically we could make Args.string()/Args.integer() work too, but it would require a lot more typescript overloads - let me know if that is preferable.