📒 Allow executable code/output to be inside a figure

[fwkoch]

This PR consolidates the logic for finding code/output cells in a block. It also reduces the strictness of this logic:

Previously the only block structure that resolved to executable code/output was:

block
 |- code
 |- output

Now, that structure is still allowed along with:

block
 |- figure
     |- code
     |- output
     |- caption

as added in jupyter-book/mystmd#760