Add dev warning when initialUiState is missing the index name

[Haroenv]

Is your feature request related to a problem? Please describe 🙏

instantsearch({
  indexName: 'my_index_name',
  initialUiState: {
    query: 'default set query',
  },
})
  .addWidgets([searchBox({})])
  .start();

Giving this does not give a warning at all, while it should.

(the correct code is:)

instantsearch({
  indexName: 'my_index_name',
  initialUiState: {
    my_index_name: {
      query: 'default set query',
    },
  },
})
  .addWidgets([searchBox({})])
  .start();

Describe the solution you'd like 🤔 A clear and concise description of what you want to happen.

A warning somewhat like this should be thrown:

[InstantSearch]: warning! You have passed the following `indexUiState`:

This is missing one of the registered indices: ${indexIds.join(', ')}.
A correct usage would be at least:

instantsearch({
  // ...
  indexName: '${rootIndexName}',
  initialUiState: {
    '${rootIndexName}': ${JSON.stringify(initialUiState, null, 2)}
  }
})

Describe alternatives you've considered ✨ A clear and concise description of any alternative solutions or features you've considered.

• I was thinking we could warn if the value of a root UiState is not an object, but that would fail with most widgets (hierarchical, refinement, menu ...) which have a key per attribute.

• We could also whitelist based on known keys of widgets, however that isn't technically an error on their part, but probably the closes

• We can also warn when none of the keys are a registered index (i.e. either root index name, or its child widgets (recursive)). This needs to happen at the init step probably.

• I don't see how to put this in the existing warning, since that's on a per-index level, so we'd need to warn as well if nothing has initialUiState, but there was a non-empty initialUiState.

• we can also warn if there are multiple keys, and the root index is missing, but that could be a false positive if there are other index widgets.

[Haroenv]

Implementation of the check which probably could work:

if (__DEV__) {
  function getIndexIds(i: IndexWidget): string[] {
    return i
      .getWidgets()
      .filter(isIndexWidget)
      .reduce<string[]>(
        (acc, curr) => [...acc, ...getIndexIds(curr)],
        [i.getIndexId()]
      );
  }

  const indexIds = Object.keys(initialUiState);
  const expectedIndexIds = getIndexIds(this.mainIndex);

  if (indexIds.every((id) => expectedIndexIds.indexOf(id) === -1)) {
    console.log('warning');
  }
}