Add labeled step like `step(name, label, fn)` so that user can access the step with label after

[lambdalisue]

For example

scenario(...)
  .step("Something interesting", "foo", () => {
    return {foo: "bar"};
  })
  .step("Use interesting things", (ctx) => {
    const obj = ctx.results.foo; // instead of ctx.previous or ctx.results[0]
    return obj;
  })
  .build();