Support "as" with best-effort in the absence of a model

[jmandel]

Currently, evaluating "path.to.choiceType as Quantity" only works when a model is available. Example:

// example.js

import fhirpath from "https://esm.sh/fhirpath"
import r4 from "https://esm.sh/fhirpath/fhir-context/r4"

const o = {"resourceType": "Observation", "valueQuantity": {"value": 123}}
const withModel = fhirpath.evaluate(o, "value as Quantity", null, r4)
const noModel   = fhirpath.evaluate(o, "value as Quantity")

console.log("With model", withModel);
console.log("No model", noModel);

$ deno run example.js 
With model [ { value: 123 } ]
No model []

It'd be very convenient if fhirpath.js could make a "best effort" at resolving "as Quantity" using FHIR conventions, even without any specific model loaded. So looking for foo.bar as Quantity could essentially act like foo.barQuantity even if no model is available.