Fix conversion of Js.t containing an alias core type

[IwanKaramazow]

[@bs.deriving {abstract: light}]
type jsProps('a) = {
  config: Js.nullable(Js.t({..} as 'a)),
  image: Js.nullable(string),
  label: Js.nullable(string),
  maxwidth: Js.nullable(int),
};

The Js.t({..} as 'a) part is problematic. There's currently no way to express this in ReScript.
{..} is the syntax for Js.t({..}). So technically {..} as 'a (rescript), represents Js.t({..)) as 'a.
However in the above code the alias sits inside the Js.t.

[chenglou]

Guess no need anymore after rescript-lang/rescript#4967