fix:typescript: set default export

[alzalabany]

javascript use module.exports = function (opts) { which is equivalent to default export in typescript;

-keypair is not exported as named export in javascript

in typescript if lib. is imported using:

• import {keypair} form 'keypair'} which cause "keypair.keypair is not a function" error
• import * as keypair from 'keypair' will throw typescript error; keypair is not a function
• import keypair from 'keypair'; will result in 'keypair doesnot have default export' error;

this pull request export keypair as default export; i kept named export as is in case it was placed there for a future reason!

[juliangruber]

Thanks for the explanation 🙌 https://github.com/juliangruber/keypair/releases/tag/v1.0.3