Typescript: add 'lastResponse' to return types

[richardm-stripe]

r? @ctrudeau-stripe @jlomas-stripe
cc @remi-stripe @stripe/api-libraries

Fixes #983

Approach:

In codegen, all the API responses that correspond to single resources now return e.g. Stripe.Response<Customer> instead of Customer. What is Stripe.Response?

export type Response<T> = T & {
  headers: {[key: string]: string};
  lastResponse: {
    requestId: string;
    statusCode: number;
    apiVersion?: string;
    idempotencyKey?: string;
    stripeAccount?: string;
  };
};

As far as I can reason, intersection types are subtypes of their constituents, so this should be non-breaking and safe.

List responses are a little trickier. I didn't change anything in the codegen, because each method definition (e.g. stripe.accounts.listExternalAccounts has a type like

/**
 * List external accounts for an account.
 */
listExternalAccounts(
  id: string,
  params?: ExternalAccountListParams,
  options?: RequestOptions
): ApiListPromise<Stripe.BankAccount | Stripe.Card>;
listExternalAccounts(
  id: string,
  options?: RequestOptions
): ApiListPromise<Stripe.BankAccount | Stripe.Card>;

so instead I redefined ApiListPromise to extends Promise<Stripe.Response<ApiList<T>>> instead of extends Promise<ApiList<T>>

I was also a bit concerned about the "intersection type of union types". In theory, Response<BankAccount | Card> should be the same as Response<BankAccount> | Response<Card>, and so when a user does a refinement (i.e. they do if (res.object === 'card') {...}), inside their if statement, resshould be aResponse`. I checked it and in practice this seems to work.
Testing strategy:

Added property accesses to the typescriptTest.js for lists and a non-list response.

[ctrudeau-stripe]

LGTM