feat(instantsearch.js): introduce `relatedProducts` widget

[sarahdayan]

This introduces the relatedProducts widget and tests it in the widget common test suite.

This PR changes a few things in how we pass templates to the UI component:

• The <RelatedProducts> UI component now defines all default templates. This lets us define them once for all flavors instead of passing them manually and risking output drifts.

• InstantSearch.js doesn't pass default templates in relatedProducts since they're defined in the UI component.

• All relatedProducts templates are passed as Fragments so we can avoid redefining components in the widget.

• https://algolia.atlassian.net/browse/FX-2814

• https://algolia.atlassian.net/browse/FX-2815

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Sandbox	Source
react-instantsearch-app	Configuration
example-react-instantsearch-default-theme	Configuration
example-react-instantsearch-next-app-dir-example	Configuration
example-vue-instantsearch-default-theme	Configuration