Declare kaleido as an optional dependency

[tpgillam]

When calling write_image on a figure, it is required that kaleido is installed in the active environment. It would be great if this dependency were declared in pyproject.toml as an optional group.

For example, here is the approach taken in pyvista:

https://github.com/pyvista/pyvista/blob/e1401a34cbd281bbe74bce8dccbb78b85ab36dc4/pyproject.toml#L40-L59

This would improve ergonomics for packages that depend on plotly, since kaleido is (from their perspective) an implementation detail. Such a package could then depend on plotly[io], for example.