fix: typescript export needs es6

[simllll]

fixes #1267

There are two issues:
1.) there is only one default export, no named export. People are assuming a named export will work fine too. Which is not direclty a bug, but a nice to have ;)
2.) The export with es5 does not work correclty, as es5 is not aware of class exports.
Test:

import { Agenda } from 'agenda';
console.log('AGENDA', Agenda);

import Agenda from 'agenda';
console.log('AGENDA', Agenda);

all should output:
AGENDA [class Agenda extends EventEmitter]
But with es5 they output:
AGENDA [Function: Agenda]

[RA80533]

The root index.js file can now be safely removed since it's no longer used as the entry point.

[simllll]

The root index.js file can now be safely removed since it's no longer used as the entry point.

Thanks @RA80533 ,pinging @koresar for final okay :)