[myst] Implement support for representing outputs of executed code cells

[agoose77]

Jupyter Book 1 is able to fold executed outputs into the document naturally following execution. The MyST engine does not yet support this, and instead leaves the data inside a black-box data field.

Suggested Actions

1. Update MyST AST to represent outputs
2. Add upgrade/downgrade routine to use this facility
3. Update execution engine to write to this AST
4. Update myst-theme to consume new outputs

As part of our "tactical upstream work", this issue tracks the implementation of this feature.

See upstream issue: jupyter-book/mystmd#1026

Definition of Done

• It should be possible to query MyST AST to interrogate per-output data