Integrate webr and Pyodide for providing data and initial values

[WardBrian]

See Proof of concepts in #26 and #27.

In both cases, the idea is that there would be a tab in the current location of data.json. This would load a scripting environment, and assigning to a magic variable called data will be used as the data.

For Python, we can use stanio to transform this into JSON. This handles a few of the edge cases in Stan's JSON format. Currently no direct equivalent exists in R, so we will just do our best

For initial values, we can similarly allow a magic variable called inits or initialize which should be set to a function that takes in the chain ID and returns initial values. This nicely side-steps the UI issues of how to support multiple initial values for the multiple chains.

[magland]

I'd like to rewrite #74 on the new main... but (discussing here with @jsoules) I won't use the routing for it, just a special popup. I think it will be helpful to develop and prototype these before figuring out how to plugin them into the UI.

[WardBrian]

Closed by #120 and moved the rest to #139