Fix type definition

[jdforsythe]

The current type definition requires esModuleInterop: true to work. Otherwise you get errors like: TypeError: (0 , keypair_1.default) is not a function

References:

• https://www.typescriptlang.org/docs/handbook/declaration-files/templates/module-d-ts.html - "Note that using export default in your .d.ts files requires esModuleInterop: true to work. If you can’t have esModuleInterop: true in your project, such as when you’re submitting a PR to Definitely Typed, you’ll have to use the export= syntax instead. This older syntax is harder to use but works everywhere."
• https://stackoverflow.com/questions/55910285/export-custom-type-definition-with-module-exports
• https://www.typescriptlang.org/docs/handbook/declaration-files/templates/module-function-d-ts.html

This change uses Declaration Merging and the export = syntax so it works with and without esModuleInterop.

import * as keypair from 'keypair';

const keys: keypair.KeypairResults = keypair();

[juliangruber]

Could someone with more typescript experience than me please review this?