[myst] Implement support for structured code-cell outputs in MyST AST

[agoose77]

The MyST engine needs some development work to add support for AST representation of code cell outputs. Right now, each code-cell has a singular Output nodes that contains many children. In practice, we need many Output nodes that will contain mostly singular children.

This is tracked upstream in jupyter-book/mystmd#1674

Suggested Steps

• 🪓 Split Output into Outputs → Output[] jupyter-book/mystmd#1671

Definition of Done

• The AST has been modified in a non-breaking way for existing consumers of MyST AST
• Future builds of MyST can simplify the AST to drop the forward-compatibility without major changes
• The AST supports per-output children, e.g. to support Markdown output parsing.