Restrict top-level autocompletion for VS Code (PyLance)

[binste]

Currently, this PR is only meant to show some code related to discussions happening in #2918, microsoft/pylance-release#3709 (reply in thread) and #2614.

As described in https://github.com/microsoft/pyright/blob/main/docs/typed-libraries.md#library-interface, I expanded the wildcard import from vegalite and added py.typed. However, selection still shows up in autocompletion although it should be considered private by PyLance as it is not mentioned in __all__:

Opening altair/__init__.py in VS Code shows that PyLance is aware that it should be private so this is somehow ignored by the code completion?

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Before this approach could be merged, we'd need to:

• Understand what implications are of adding py.typed. Altair is far off from being “type complete” as it is described here. Maybe what we want to achieve is possible without adding py.typed for now?
• Automate expansion of from .vegalite import *