Interactive Plot Inputs

[OAGr]

We've previously discussed the idea of plots. #297

It could be also really nice to have interactive components, like with Observable. They would be a bit weird, but could be neat.

Possible interfaces, in Squiggle:

a = normal($inputMean, $inputStdev)

[
  interactiveSlider({name: inputMean, min: 0, max: 20, default: 5}), 
  interactiveSlider({name: inputStdev, min: 0, max: 20, default: 5}),
  a
]

The obvious ideas for interaction are 1d sliders and 2d sliders (can hover over any xy coordinate, to adjust both coordinates).

This is clearly kind of weird because one Squiggle file is declaring $inputMean after calling it! That would be fine with Observable, and it might be safe if it were applied carefully. I think the logic could all be done in React.