Ensure internal classes used for style `:host(.<class>)` are not overridden by `className`

[fcollonval]

When using the treeview in JupyterLab, we saw that some tree item were not getting the expected nested class that is handled within the component.

This solves it as well as potential similar issues on other components.

[github-actions[bot]]

:point_left: Launch a Binder on branch jupyterlab-contrib/jupyter-ui-toolkit/fix/tree-item-class