d2oracle: set on imported nested board

[alixander]

e.g.

cat.d2

a

scenarios: {
  b: {
    meow
  }
}

dog.d2

...@cat

and then you do d2oracle.Set on dog.d2 with board path b, changing meow opacity or something.

---

Options:

1. d2oracle needs to do like a mkdir -p equivalent, where the board path maps are created if not found
2. d2oracle can define it as a flat map set. e.g. scenarios.b.meow.style.opacity: 0.1

I think option (2) is right, since that's the behavior for similar ops (put in map if exists, otherwise append flat map line)