test: generate index test for clients

[kuhe]

This adds a smithy-build config option called generateIndexTests: boolean, default false.

When set, it generates the index-types.spec and index-objects.spec files which can be run in the client script test:index only needing tsc and node.

This allows us to opt into running an API surface test when modifying things like how package components are exported, in the future updates where some exports are turned into types-only, or changes in what gets generated.

Example:

types test:

// smithy-typescript generated code
export type {
  XYZServiceClient,
  XYZService,
  GetNumbersCommand,
  TradeEventStreamCommand,
  Alpha,
  GetNumbersRequest,
  GetNumbersResponse,
  TradeEvents,
  TradeEventStreamRequest,
  TradeEventStreamResponse,
  Unit,
  XYZServiceServiceException,
} from "../dist-types/index.d";

runtime test:

import {
  GetNumbersCommand,
  TradeEventStreamCommand,
  XYZService,
  XYZServiceClient,
  XYZServiceServiceException,
} from "../dist-cjs/index.js";
import assert from "node:assert";
// clients
assert(typeof XYZServiceClient === "function");
assert(typeof XYZService === "function");
// commands
assert(typeof GetNumbersCommand === "function");
assert(typeof TradeEventStreamCommand === "function");
// errors
assert(XYZServiceServiceException.prototype instanceof Error);
console.log(`XYZService index test passed.`);