Include more JS primitives for Untagged variants

[nojaf]

I'm trying to model blobParts of https://developer.mozilla.org/en-US/docs/Web/API/Blob/Blob#blobparts for the new WebAPI bindings.

@unboxed
type blobPart =
  | String(string)
  | Blob(blob)
  | ArrayBuffer(ArrayBuffer.t)
  | TypedArray(TypedArray.t<int>)

However, I cannot use ArrayBuffer.t and TypedArray.t in combination with the @unboxed.
Could some of these be extended?

[nojaf]

I'm hitting the limitations of @unboxed again for https://github.com/rescript-lang/experimental-rescript-webapi/pull/7/files#r1839785692

Modelling https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/fillStyle

It should be something like

@unboxed
type fillStyle =
  | String(string)
  | Gradient(canvasGradient)
  | Pattern(canvasPattern)

It would be great if I can input some sort of check for the unboxed via attribute, something like:

| @check("$0 instanceof CanvasGradient") Gradient(canvasGradient) // where $0 is the object in place.