Deprecate Exported Enum Values in JS

[Eyas]

See #131 for a justification/discussion.

We'd rather users use:

const kneeReplacement: MedicalProcedure = {
  "@type": "MedicalProcedure",
  "procedureType": "PercutaneousProcedure", // or "https://schema.org/PercutaneousProcedure"
};

We already accept these as constant string literals, but now we'll remove the alternative, such as:

MedicalProcedureType.PercutaneousProcedure