Add separate type file for esm per TypeScript guidelines

[shadowspawn]

Pull Request

Problem

TypeScript wants a type definition file which matches the implementation file, like foo.js + foo.d.ts, bar.mjs + bar.d.mts.

See #1880 for more detail.

Solution

Adding the file was easy. Working out how to just reexport the existing types was tricky! I am using:

// This is a bit indirect. There is not an index.js, but TypeScript will look for index.d.ts for types.
export * from './index.js';

The direct ./esm.mjs entry is stale, but tidy it up too.

ChangeLog

• changed: refine "types" exports for ESM to follow TypeScript guidelines

[abetomo]

Awesome!