Allow `<details>` elements in parameter panes

[Vectornaut]

Letting users hide and show parameters can make it easier to navigate visualizers and sequences with complicated parameter panes. We currently do this using special parameters that only affect the visibility of other parameters (for example, the Morphing controls checkbox in the Turtle visualizer). This affordance has several downsides:

• Easy to confuse with parameters that actually affect the visualizer or sequence.
• Hiding or showing parameters restarts the specimen, even though none of the parameters affecting the specimen have changed.

Both of these problems could be solved by allowing <details> elements in parameter panes.

[gwhitney]

There is a key design question here: how can the presence of such elements and which parameters will/won't be hidden/revealed when the <details> is activated/deactivated, be indicated in the parameters object? I think we need a specific proposal before this issue becomes truly actionable (hence it is not scheduled for any particular milestone, at the moment).