Make plot_code a Parameter

## Request

Change `plot_code` from a function to a parameter. Or document how to automatically update the plot code in some output cell in a notebook.

## Motivation

I love that the fact that I can export my `explorer` work as code.

But I would like the code to be able to just update automatically because

- Its friction that I have to rerun a cell to get the code.
- The output of the `plot_code()` might not be in sync with the explorer selections if I ran that function some time ago.
- I want to be able to easily deploy an explorer Panel data app to my users such that the starting point for a visualization is just a web page that can take us a long way, then we export the code and continue if/ when needed.