This replicates what mxgraph examples previously provided: the source code directly accessible within the example.
It allows copy/paste for reuse in user applications and should improve the adoption.
The code used to manage the controls is easily identifiable and can help highlight certain parts of the example (ContextMenu, RubberBand, ...).
Also bump storybook from 6.4.0 to 6.4.13.
Remove the extra NODE_OPTIONS in storybook server start script that generated an
error.
Remove cross-env devDependency which is then not used anymore.
Fix storybook deprecation warning (static dirs)