[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 work paves the way for properly supporting Markdown outputs in Jupyter Notebooks.

This is tracked upstream in jupyter-book/mystmd#1903 and jupyter-book/myst-theme#571

Definition of Done

• A PR has been authored such that the MyST engine can successfully represent per-output ASTs from execution.
  • This does not require us to yet develop that feature with Markdown parsing support
• A PR has been authored such that the MyST themes can successfully render outputs in the new AST format.
• A set of examples to test outputs behaviour has been developed