Support extracting transformed chart data using VegaFusion (#3081)

* Port transformed_data functionality from VegaFusion

* Add initial transformed_data tests

* Test exclude flag to transformed_data

* chart.transformed_data -> chart._transformed_data. Make method internal while still experimental

* Add VegaFusion as dev dependency

* Add better error message when VegaFusion is not installed

* Make utils.transformed_data internal, use absolute imports

* Use DataFrameLike protocol for the transformed_data signature

The returned DataFrames can be arrow or Polars as well if those are used as the input to the Chart.

* Add NotImplementedError for RepeatChart

* Use Chart._get_name to name subcharts

* Protocol is available in Python 3.8

* Make DataFrameLike private for now

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Co-authored-by: Stefan Binder <[email protected]>