expand the inspector if its previous version was expanded

[Fil]

We save the current inspector's expansion state in root.inspectorExpanded, and restore it when we create a new inspector, by passing a div that contains a span with the expected "observablehq--expanded" class.

for now inspectorExpanded is true if defined — this should be revised more thoroughly when we fix observablehq/inspector#12

closes #1458

[mbostock]

We should avoid using delete and instead represent inspector state consistently by initializing it when the root node is attached (i.e., in define).

Also, we should handle the multiple displayed items case better — we should err towards collapsing rather than expanding.

(I think technically we could avoid storing any additional state on the root because it could be part of the display closure, but that’s just an optimization I guess.)

[Fil]

my test.md bench:

```js
const value = view(Inputs.button("reset"));
```

```js
const foo = {name: "foo", value};
const bar = {name: "bar", more: {value}};
```

```js
foo
```

```js
if (Math.random() > 0.5) display(foo);
display(bar);
```

```js
Math.random() > 0.5 ? foo : bar
```

[mbostock]

🚀