Replace event-emitter.js by mitt

[lucleray]

This PR aims at replacing next-server/lib/event-emitter.js by mitt.

Fix #4908

event-emitter.js is ~400 bytes gzipped vs mitt is 200 bytes

[timneutkens]

cc @DullReferenceException

[DullReferenceException]

Isn't this a breaking change? It seems like doing a return this; in mitt would be a small change that would avoid surprising those used to node's EventEmitter.

[lucleray]

I'm afraid it would make mitt much bigger in size, because mitt is not a class but a function.

[DullReferenceException]

That doesn't matter; mitt() returns an object, so this would refer to that object, and doing a return this inside of on and off would allow for the same type of method chaining.

[lucleray]

You're right, I forgot about that 😅

@timneutkens Should we return this in on and off, ?

It adds ~25 bytes in the minified code (not gzipped).

[timneutkens]

I guess it's fine to add it, however, I'm curious what @developit thinks.

[developit]

Mitt doesn't currently return this because the context of the method isn't guaranteed:

const { emit, on, off } = mitt()

on('foo', console.log)
    .emit('foo')

The value of this when on is invoked is window/undefined.

This is also true when mixing mitt into an object:

router = Object.assign(Router(), mitt())
router.on() // this is Router instance

If chaining is important as part of the public API here, I think it's best to implement the return semantics here.