Fix typings compatibility with p-event

[papb]

With the release of 0.8.0, the following no longer type-checks:

import pEvent = require('p-event');

const ee = new Emittery();
pEvent.iterator(ee, 'data', {
	resolutionEvents: ['finish']
});

This PR fixes this.

[sindresorhus]

// @airhorns

[airhorns]

If I understand correctly, this issue arises because TypeScript can't assign an Emittery to the p-event.Emitter type before this change, and it can't do that because neither overloaded on/off function cover the event name type all together, despite them covering it when put together?

[papb]

Hi @airhorns, thank you for taking a look, to be honest I was in such a hurry that I didn't check exactly what was the reason for it to not be working; I am not sure if it is just a TypeScript limitation. I happened to figure out this fix by luck, I agree with you that the previous overloads seem equivalent to what I did now.

[papb]

Replaced by #73