Allow users to provide type hints to Value.from()

[zslayton]

Currently, Value.from converts all JS arrays into ion.dom.Lists and all Uint8Arrays into ion.dom.Blobs. Users hoping to create ion.dom.SExpression or ion.dom.Clob have to instantiate them manually, which is much more tedious.

We should modify Value.from to accept an optional expected type hint. This is trivial for handling the top level value:

let jsValue = [1, 2, 3];
let annotations = ["foo", "bar"];
let list = Value.from(jsValue, annotations);
// foo::bar::[1, 2, 3]

let expectedType = IonTypes.SEXP;
let sexp = Value.from(jsValue, annotations, expectedType);
// foo::bar::(1 2 3)

However, it's unclear how to specify the expected outcome of converting nested JS values. Would the expectedType hint apply to ALL Arrays and Uint8Arrays encountered during conversion? Is there a better API?

[donpdonp]

Similarly I was hoping to be able to say ion.dom.Value.from(IonTypes.Date, "2021-01-1") and have the library convert the date string into an Ion date value.