feat(js): expose GetSources type

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Summary

Expose GetSources type in autocomplete-js, which can be useful when extracting the function from the Autocomplete instance.

Used in the panel-placement example: #549

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Sandbox	Source
@algolia/autocomplete-example-github-repositories-custom-plugin	Configuration
@algolia/autocomplete-example-multiple-datasets-with-headers	Configuration
@algolia/autocomplete-example-playground	Configuration
@algolia/autocomplete-example-query-suggestion-with-categories	Configuration
@algolia/autocomplete-example-query-suggestions-with-hits	Configuration
@algolia/autocomplete-example-query-suggestions-with-inline-categories	Configuration
@algolia/autocomplete-example-query-suggestions-with-recent-searches-and-categories	Configuration
@algolia/autocomplete-example-query-suggestions-with-recent-searches	Configuration
@algolia/autocomplete-example-query-suggestions	Configuration
@algolia/autocomplete-example-react-renderer	Configuration