🪓 Split Output into Outputs -> Output[]

[agoose77]

This PR is an alternative to #1661 that attempts to solve #1026. It does the following:

1. Introduces a distinction between internal AST and external AST.
2. Upgrades external AST into internal AST (e.g. pulling in xref).
3. Downgrades internal AST to external AST (e.g. on writing to disk).

Tip

Internal AST is the AST that our transforms (and plugins) operate upon.
External AST is the AST that we publish to e.g. the web.
The factoring of our AST into two "branches" means that we can defer breaking changes to an arbitrary point in the future.

New Internal AST Form

The internal AST has the following form:

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

type Outputs = {
  type: "outputs";
  children: Output[];
  visibility: ...;
  identifier: ...;
  html_id: ...;
}

type Output = {
  type: "output";
  children: GenericNode[];
  jupyter_data: IOutput;
}

New External AST Form

type CodeBlock = {
  type: "block";
  kind: "notebook-code",
  children: [
    Code,
    Output
  ]
}
type Output = {
  type: "output";
  children: GenericNode[];
  data: IOutput[];
  _future_ast: Outputs
}

In this PR, I don't break our published AST at all by storing a "replacement" AST in a new node field.

Note

The pattern of _future_ast replacing the current node seems like it might be quite useful. A way to systematically define forwards-compatible AST.

Closes #1674

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Latest commit: 7ef0e1e

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