* feat(ts): allow custom ui state and route state in routing

While I couldn't find a way to make InstantSearch itself generic (this gets passed to many places, which then loses generic), using the routing middleware directly is possible like this now

```ts
import instantsearch from 'instantsearch.js/es'
import { history } from 'instantsearch.js/es/lib/routers';
import { createRouterMiddleware } from 'instantsearch.js/es/middlewares';
import { StateMapping, UiState } from 'instantsearch.js/es/types';

type SwagIndexUiState = { swag: boolean };
type SwagUiState = { [indexName: string]: SwagIndexUiState };

const stateMapping: StateMapping<UiState & SwagUiState, SwagUiState> = {
  stateToRoute(uiState) {
    return Object.keys(uiState).reduce(
      (state, indexId) => ({
        ...state,
        [indexId]: { swag: uiState[indexId].swag },
      }),
      {}
    );
  },

  routeToState(routeState = {}) {
    return Object.keys(routeState).reduce(
      (state, indexId) => ({
        ...state,
        [indexId]: routeState[indexId],
      }),
      {}
    );
  },
};

const search = instantsearch();

search.use(
  createRouterMiddleware<UiState & SwagUiState, SwagUiState>({
    router: history(),
    stateMapping,
  })
);
search.addWidgets([instantsearch.widgets.hits({ container })]);
```

* actually genericificate InstantSearch

* already inferred so no need

* Apply suggestions from code review

* address feedback

* better comment

an example of how it's different is:

```ts
import { Expand, UiState } from './types';

type mix = UiState & { [index: string]: { swag?: boolean } };
type mixExpand = Expand<mix>;

// @ts-expect-error
const initialUiState: mix = {
  someIndex: {
    swag: true,
  },
};

const initialUiStateExpanded: mixExpand = {
  someIndex: {
    swag: true,
  },
};
```

fix yarn link

also removes the file for "main" as it's inconsistent with index.es.ts

* chore: release v4.26.0

* Update CHANGELOG.md

Co-authored-by: Haroen Viaene <hello@haroen.me>