Mark if let as experimental

[IwanKaramazow]

if let Error(x) = result {
  Js.log("The sky is red")
} else if let Ok(y) = result {
  Js.log("The sky is blue")
} else {
  Js.log("no sky")
}

[chenglou]

Yeah this needs a bit more caution. Since it's additive we can postpone it until we work out the stance on the compiler warnings, the actual behavior of else if let, how to prevent abuse of this syntax, etc.

[chenglou]

Also this flexes the error messages system so much. This is great