Enabling preview button on raw .py files in Jupyter Lab

[alexveden]

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

I'm using plain text notebooks rendered by Jupytext lib, which are essentially raw python files with %% delimiters. But I've found that Panel Preview button is only available for '.ipynb' files. Could you add such functionality into plugin?

Describe the solution you'd like

Enabling registering preview plugin button for all .py files.

Describe alternatives you've considered

Also, it might be useful to have field in options for other file types, for example Jupytext is capable to render .md as notebooks too.

Additional context

Thanks