Prefer completing siblings rather than more deeply nested entries

[svenstaro]

If you have a document like this:

{
  "a": {
    "deep": 1
  },
  "b": 2
}

then the completion will go

.a
.a.deep
.b

but I think it makes more sense to complete to entries at the same level first, so

.a
.b
.a.deep