Add support for a node / list of nodes at fields other than `children`

[wooorm]

Subject of the feature

As unist is slowly moving a tiny bit away from just content, it starts to make more sense to add support for nodes in other places than children.

Problem

Thinking about JSX elements in Markdown which have open and close fields leading to jsxTag nodes.

Expected behaviour

Something like this maybe?

root[2]
├─ node [id="a"]
└─ parent[1]
   ┊  someKey: node [id="b"]
   ┊  otherKey[2]:
   ┊  ├─ node [id="c"]
   ┊  └─ node [id="d"]
   └─ node [id="e"]

Alternatives

Currently it’s something like:

root[2]
├─ node [id="a"]
└─ parent[1] [someKey={type: "node","id":"b"}][otherKey=[{type: "node","id":"c"}, {type: "node","id":"d"}]]
   └─ node [id="e"]