feat(routing): add a "single index" compatibility mode

[Haroenv]

This mode has as goal to flatten the URL structure to avoid the indexName of the root index in the URL when there's only one URL.

Usage:

import { history } from 'instantsearch.js/lib/router';
import { singleIndex } from 'instantsearch.js/lib/stateMappings';

instantsearch({
  // ...
  indexName: "blabla",
  routing: {
    router: history(),
    stateMapping: singleIndex("blabla"),
  },
});

Right now it's not yet possible to use this router in TS because InstantSearch itself isn't generic, and trying to make this happen was causing me a cascade of Generic issues. For now you need to cast it (stateMapping: singleIndex("blabla") as StateMapping<UiState>).

BREAKING CHANGE: Default URLs have changed, and now will contain the full state, which includes the index name. You can use the old behaviour by using the singleIndex stateMapping, and passing the index to synchronise in the argument.

https://algolia.atlassian.net/browse/IFW-941

[francoischalifour]

• Are we all aligned that we want this feature to be opt-in?
• Do we want to reiterate on our router vocabulary?

[Haroenv]

I found opt-in the most elegant. We could make a version which has access to the instance & the tree, but the logic seemed too inflexible to me

[tkrugg]

Currently I can't test this because it's complaining about missing searchOnlyWithDerivedHelpers
I think #3885 adds it, would it make sense to rebase ?