Switch from os.path to pathlib in MeshCat visualizer

[stephane-caron]

This is a minor proposal to remove the use of os.path in the MeshCat visualizer and only use pathlib. It also adds some type annotations (although we don't use mypy or another static Python type checker yet).

[github-actions]

👋 Hi,
This is a reminder message to assign an extra build label to this Pull Request if needed.
By default, this PR will be build with minimal build options (URDF support and Python bindings)
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The Pinocchio development team.