Pass component instance as second argument in computed setters

[posva]

What problem does this feature solve?

Allow the usage of arrow functions in copmuted setters
From #7688

computed: {
  value: {
    get: vm => vm.someValue,
    set: (val, vm) => vm.someValue = val
  }
}

What does the proposed API look like?

computed: {
  value: {
    get: vm => vm.someValue,
    set (val, vm) {
      this === vm // true
    }
  }
}