Support for VegaFusion data transformer in Vega pane

[jurriaanpro]

Is your feature request related to a problem? Please describe.

https://panel.holoviz.org/reference/panes/Vega.html#altair

I'm trying to plot an altair chart with a large amount of datapoints which makes it throw a MaxRowsError.

    raise MaxRowsError(
altair.utils.data.MaxRowsError: The number of rows in your dataset is greater than the maximum allowed (20000).

Try enabling the VegaFusion data transformer which raises this limit by pre-evaluating data
transformations in Python.
    >> import altair as alt
    >> alt.data_transformers.enable("vegafusion")

Or, see https://altair-viz.github.io/user_guide/large_datasets.html for additional information
on how to plot large datasets.

Following the URL in the error I have installed and enabled the vegafusion data transformer, but then I run into

File "<snip>/.venv/lib/python3.11/site-packages/panel/pane/vega.py", line 79, in _get_schema_version
    schema = obj.to_dict().get('$schema', '')
             ^^^^^^^^^^^^^
  File "<snip>/.venv/lib/python3.11/site-packages/altair/vegalite/v5/api.py", line 984, in to_dict
    raise ValueError(
ValueError: When the "vegafusion" data transformer is enabled, the 
to_dict() and to_json() chart methods must be called with format="vega". 
For example: 
    >>> chart.to_dict(format="vega")
    >>> chart.to_json(format="vega")

Describe the solution you'd like

I would like the Vega widget in Panel to detect whether the VegaFusion data transformer is enabled, and pass the required format="vega" parameter to the to_dict calls.

https://github.com/holoviz/panel/blob/main/panel/pane/vega.py#L82
https://github.com/holoviz/panel/blob/main/panel/pane/vega.py#L127

This might be used as inspiration: https://github.com/vega/altair/blob/main/altair/utils/_vegafusion_data.py#L282,L287

Describe alternatives you've considered

Reducing the amount of datapoints is a possibility, but reduces usability.