text: Title: Support for VegaFusion data transformer in Vega pane 
  Body: #### 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.