fix(ts): expand UiState mix

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,
  },
};
```

src/index.es.ts

@@ -1,4 +1,4 @@
-import { InstantSearchOptions, UiState } from './types';
+import { Expand, InstantSearchOptions, UiState } from './types';
 import InstantSearch from './lib/InstantSearch';
 import version from './lib/version';
 import {
@@ -15,7 +15,7 @@ const instantsearch = <
   TUiState = Record<string, unknown>,
   TRouteState = TUiState
 >(
-  options: InstantSearchOptions<UiState & TUiState, TRouteState>
+  options: InstantSearchOptions<Expand<UiState & TUiState>, TRouteState>
 ) => new InstantSearch(options);
 
 instantsearch.version = version;

src/lib/InstantSearch.ts

@@ -112,7 +112,7 @@ export type InstantSearchOptions<
    * for the first search. To unconditionally pass additional parameters to the
    * Algolia API, take a look at the `configure` widget.
    */
-  initialUiState?: UiState;
+  initialUiState?: TUiState;
 
   /**
    * Time before a search is considered stalled. The default is 200ms

src/lib/main.ts

@@ -1,3 +1,5 @@
+import { Expand, InstantSearchOptions, UiState } from '../types';
+
 import InstantSearch from './InstantSearch';
 import version from './version';
 
@@ -9,7 +11,6 @@ import * as middlewares from '../middlewares/index';
 import * as routers from './routers/index';
 import * as stateMappings from './stateMappings/index';
 import { createInfiniteHitsSessionStorageCache } from './infiniteHitsCache/index';
-import { InstantSearchOptions, UiState } from '../types';
 
 /**
  * InstantSearch is the main component of InstantSearch.js. This object
@@ -32,7 +33,7 @@ const instantsearch = <
   TUiState = Record<string, unknown>,
   TRouteState = TUiState
 >(
-  options: InstantSearchOptions<UiState & TUiState, TRouteState>
+  options: InstantSearchOptions<Expand<UiState & TUiState>, TRouteState>
 ) => new InstantSearch(options);
 
 instantsearch.routers = routers;