Declare `kaleido` as an optional dependency

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.

從 pyproject.toml 開始，將其選用相依項設定與連結的 PyVista 範例進行比較。在一個選用群組中宣告 kaleido，讓套件可以相依於 plotly[io]，然後驗證相依項中繼資料是否正確公開了該 extra。