Inspector v2: Stub out stats pane

[ryantrem]

In this PR, I'm stubbing out a new stats pane, which includes:

1. Factoring out an AccordionPane react component from the PropertiesPane component. The idea is that we will have several panes that will have the same basic accordion UI, and each can support the same type of extensibility as properties.
2. PropertiesPane/PropertiesService are slightly more complicated that the other panes in that the "context" of the pane can have different types (e.g. a Mesh or a Light). Given this, I kept the predicate stuff specific to Properties. Other panes like Stats can have a single type that is always the same (like Scene) and be a little simpler.
3. Added a new StatsPane/StatsService that otherwise follow the same model as properties.

One other difference that I am trying with StatsService is to not bother having separate service definitions for the default/built-in content of the status pane. Instead it is defined directly in the StatsService, but new service definitions could be defined to extend the default content of the stats pane. This was based off a suggestion @deltakosh had in the original inspector v2 PR.

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