Remove onAny generic

[novemberborn]

Before, you could do:

import Emittery = require('emittery')

const e = new Emittery.Typed<{foo: 'bar'}, 'baz'>()
e.onAny<'foo'>((eventName, eventData) => {
})

This is incorrect, since e.emit('baz') is allowed.

Note that TypeScript can't express the type of eventData based on eventName. Thus eventData will be the union of 'bar' | undefined. I guess it's an encouragement for adding regular listeners instead.

I also noticed that new Emittery.Typed<{}, true>() was allowed. Empty event names must still be strings.

/cc @dinoboff.