This PR is an alternative to #1671 that attempts to solve #1026 by relaxing the 1:1 constraint on Block:Output. This PR:

1. Converts from outputs with output.data.length === N into N outputs with output.data.length === 1.

New AST Form

type CodeBlock = {
  type: "block";
  kind: "notebook-code",
  children: [
    Code,
    Output,
    ...,
    Output
  ]
}

type Output = {
  type: "output";
  children: GenericNode[];
  visibility: ...;
  data: IOutput[1];
}

As this changes our AST structure, it's a "breaking change". In-person discussions will be held to explore this.

See the edit history for this description to learn how we arrived here.

Closes #1674