Explore support IPyWidgets using the new Jupyter Lab App framework

[DonJayamanne]

Basically load widgets from labextensions folder when possible.
Reason: Some widgets only work in lab environments, and given the fact that widgets placed in nbextensions are loaded only by older versions of Jupyter (Notebook) and VS Code, its highly likely that more and more widgets will either not test against hose environments or will not support them.

here's a good example #15757 (comment)