Define `vl-convert-python` dependency outside of `altair[all]`

### What is your suggestion?

The `vl-convert-python` package is necessary for saving plots in PNG / SVG / PDF formats. I believe this is a rather critical function, so I'm wondering why `vl-convert-python` needs to be manually installed for this capability. (I see that `altair[all]` is an alternative, but I think `vega_datasets`, etc. are less critical than the save functionality.)



`vl-convert-python` has zero dependencies. So I think this would be a simple addition. Maybe that is the reason Is this dependency optional because the built wheels are ~30 MB? Could this size be reduced?



Thanks so much for your consideration!

### Have you considered any alternative solutions?

Alternatively, could this (at least) be added to a new optional dependency group `altair[save]`, as suggested [here](https://github.com/vega/altair/issues/3309#issuecomment-1953600671)?

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