doc: replace util.inherits() with es6 classes extends

[eljefedelrodeodeljefe]

util.inherits() is probably long unloved. With full class support it would be worth thinking of slowly deprecating it as weakly suggested in #4179 by @bnoordhuis .

Since this would strongly promote the class keyword for the JS world we would need to think about whether we want to do this and in which form. Personally I am not a big fan of it in general - especially since es6 proposed style would have methods on a non 0 indentation level. So we could think about the following:

cc @nodejs/documentation @jasnell

const EventEmitter = require('events')

class PureES6 extends EventEmitter {
  constructor() {
    super()
  }

  echo(val) {
    console.log(val)
  }
}

let es6 = new PureES6()
es6.echo('A Value')


class Mixed extends EventEmitter {
  constructor() {
    super()
  }

  _echo(val) {
    console.log(val)
  }
}

Mixed.prototype.echo = function echo (val) {
  this._echo(val)
}

let mixed = new Mixed()
mixed.echo('A Value')

[jasnell]

util.inherits() is unfortunately going to be one of those warts that lives forever since it's so widely used. Unfortunately ES6 classes aren't much better ;-) ... it would likely be a good idea to begin slowly modernizing the examples in the docs but outright replacing likely isn't an option.

[eljefedelrodeodeljefe]

Yep, sure :-(. One upside would be that extends supposedly keeps the prototype chain as discussed in #4179.
Do you have any preference in style? Having it on the first indentation or the second level?

[jasnell]

This... always ;-)

class Foo extends Bar {
  constructor() {
  }

  baz() {
  }
}