Warnings about private fields being exposed on generated types

[zzorba]

Looks like with recent code, typescript is complaining about some fields getting exposed.

Doing edits locally, changing the macro of macros.ts to the following:

    typeof {{ decl_type_name }}[keyof Omit<typeof {{ decl_type_name }}, 'instanceOf' | '__uniffiTypeName' | '__variant' | '__variantName'>]

Seemed to fix the problem.

[zzorba]

@jhugman this is the issue we discussed, I do think moving these off the super class is the right course of action.

I probably won't have time to work on it for the next couple weeks, so we can see who gets to it first.