async package should distribute both cjs and esm

There is no real need for `async-es` package. It ends up complicating `async` usage downstream especially for projects which emit both cjs and esm. And to complicate things, `async-es` has a `main` entry which points to esm (without indicating `type: 'module'`), so it can't be used for cjs scenarios.

Recommended:

Update the `async` package.json with:

1. `main` pointing to commonjs. (this is only for backwards compatibility.)
2. `module` pointing to esm. (this is only for backwards compatibility.)
3. `exports` map explicitly listing the valid imports and the cjs/esm/d.ts files to resolve to.