proposal: builtins: add zero

[earthboundkid]

Every individual type can be compared to its zero value (func() == nil, for example), but there is no way to write this code today without using package reflect:

func IsZero[T any](t T) bool {
  return t == zero
}

It must instead be:

func IsZero[T any](t T) bool {
  return reflect.ValueOf(&t).Elem().IsZero()
}

I propose adding a built in constant zero that can be tested for equality with any type.

It could also be returned, as in

if err != nil {
  return zero, err
}

[seankhliao]

Duplicate of #35966

[earthboundkid]

I searched Github for "universal zero" and that issue did not come up. :-/