Maintain deep expanded state.

[mbostock]

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From observablehq/notebook#201:

Inspector collapse/expand state is maintained on the top level, but not when those inspectors are nested. Ideally we can maintain as much expanded/collapsed state as possible: clearest way to do this would be to maintain a list of expanded keypaths, and expand them when there's new output.

[mbostock]

This has been fixed in Observable Framework (1.10.0) but we haven’t implemented a fix for notebooks yet.