Run base constructor then blocks on the derived instance #5272

On JS, TS and Python, Fable can't inherit from a class whose explicit constructor has a self identifier, and a class with a base class can't have one. Once #5235 merges, both are compile errors, as they are on main. .NET prints true and 2.

let saved = ResizeArray<obj>()

type Base =
    val Name: string
    new (name: string) as self = { Name = name } then saved.Add(box self)

type Derived =
    inherit Base
    val X: int
    new () = { inherit Base("d"); X = 1 }

type Counter =
    val mutable Count: int
    new (c: int) as self = { Count = c } then self.Bump()
    member this.Bump() = this.Count <- this.Count + 1

type CounterChild =
    inherit Counter
    val X: int
    new () = { inherit Counter(1); X = 1 }

let d = Derived()
printfn "%b" (obj.ReferenceEquals(saved.[0], d))
printfn "%d" (CounterChild()).Count

On the #5235 branch, both derived classes fail with:

error FABLE: Program.Base has a constructor with a self identifier (`new() as x`), which Fable does not support inheriting from.

Both programs are valid F#. Supporting them means running the base then block on the derived instance. One idea, untested, is to split explicit constructors into separate allocate and initialize steps.

An earlier version of #5235 built a separate base object and copied its fields into the derived one. That was dropped because it gave wrong output for: curried function fields in the derived class, a base behind a .fsi, an [<AttachMembers>] base, and leaking self. It's in the branch history at 2a69b6a, and the tests from that version list the shapes a real fix has to handle.