Fix subtyping relationship in extended data

[mlhaufe]

const fooData = data({ Foo: {}, ... })

const barData = data({ [extend]: fooData, Bar: {}, ... })

barData[baseVariant] === fooData

This should be equal to barData. The issue and challenge is here:

// data.mjs

factory = dataDef[extend] ? Object.create(dataDef[extend]) :
    Object.assign(Object.create(protoFactory), { [baseVariant]: protoVariant })

Ideally, the fix would just be:

factory = Object.assign(Object.create(dataDef[extend] ?? protoFactory), {
    [baseVariant]: protoVariant
})

But dataDef[extend] is frozen.

[mlhaufe]

Fixed as part of #63