Add playground example that allows for compiling+inspecting a model in the browser

[chrispcampbell]

As mentioned in #437, once we have the ability to generate pure JavaScript code, it will become possible to add a new example app that demonstrates compiling a model entirely in the browser (client-side only). This can be useful for people who are new to SDEverywhere and want to evaluate whether their model works without having to install and understand SDEverywhere's tools.

I'm envisioning a REPL / playground type page that has a text box where you can paste an mdl file, and then it gets compiled to a JS model immediately. Other possible features:

• a second text box can show the generated code or any compilation errors
• a list box can show all the variables in the model
• selecting a variable can show a BoT graph of that variable
• for constant values, we can show a text field or slider that overrides that value (like a normal model input) and runs the model

We could host this page on https://sdeverywhere.org so that users could try out SDEverywhere without installing any software locally.