feat(emptyStates): implements empty source template and renderEmpty method

[shortcuts]

• If the empty source template or the renderEmpty method is defined, it will be triggered when no results are returned.
  When both are defined, the empty source template is prioritized.
• getItemCountsCount has also been moved to the autocomplete-shared folder since it's now used when calling createAutocomplete

Implementation

empty source template:
Works as the footer or header source template.

autocomplete({
    // ...
  getSources() {
    return [
      {
        getItems({ query }) {
          return getAlgoliaHits({
              // ...
          });
        },
        templates: {
          item({ item }) {
            return reverseHighlightHit({ hit: item, attribute: 'query' });
          },
          empty() {
            return 'No results';
          },
        },
      },
    ];
  },
});

renderEmpty method:
Works as the render method.

autocomplete({
    // ...
  renderEmpty({ root }) {
    const div = document.createElement('div');

    div.innerHTML = 'No results';
    root.appendChild(div);
  },
});

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 95298cf:

Sandbox	Source
@algolia/js-example	Configuration
@algolia/react-renderer-example	Configuration

[francoischalifour]

Great job!