feat(react): export InstantSearchApi type

[francoischalifour]

As discussed internally, exporting the InstantSearchApi is handy in consumers app to make sure that types are fully consistent.

As an example, there are a few unintended bugs on the dashboard search because they use their own SearchResults type for useInstantSearch().results, and they assumed that it can be null when loading. But it doesn't, RISHooks always exposes a fallback results object.

Consumer apps will now be able to rely on things like InstantSearchApi['results'] to minimize this kind of errors.

I also defaulted the generic to UiState so that it can be used just like useInstantSearch() which is already defaulted.

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Sandbox	Source
InstantSearch.js	Configuration
react-instantsearch-app	Configuration
example-react-instantsearch-hooks-default-theme	Configuration
example-vue-instantsearch-default-theme	Configuration