feat: introduce initialUiState option (#4074)

**Summary**

This PR introduces the `initialUiState` option at the `InstantSearch` level. The option allows users to provide an `uiState` used for the initial request. Only for the **initial request**. I've chosen to name the option `initialXXX` rather than `uiState` to have a clear intent. At some point, we might want to introduce the concept of controlled state (like React InstantSearch) that could use the option `uiState`.

At the moment the type definitions are incorrect. We have only one type that defines the `uiState` but we have to define two different types. The first is one is the "global" `uiState` and the second one is the "local" to each index. The "global" `uiState` is built on multiple "local" `uiState`. We'll update the definition in a separate PR.


**Usage**

```js
const search = instantsearch({
  // ...
  initialUiState: {
    instant_search: {
      query: "Apple"
    },
    instant_search_price_asc: {
      refinementList: {
        brand: ["Apple"]
      }
    }
  }
});
```

Checkout the example on [Storybook](https://deploy-preview-4074--instantsearchjs.netlify.com/stories/?path=/story/instantsearch--with-initialuistate).

Author: samouss

src/lib/InstantSearch.ts

@@ -103,6 +103,14 @@ export type InstantSearchOptions<TRouteState = UiState> = {
    */
   searchParameters?: PlainSearchParameters;
 
+  /**
+   * Injects a `uiState` to the `instantsearch` instance. You can use this option
+   * to provide an initial state to a widget. Note that the state is only used
+   * for the first search. To unconditionally pass additional parameters to the
+   * Algolia API, take a look at the `configure` widget.
+   */
+  initialUiState?: UiState;
+
   /**
    * Time before a search is considered stalled. The default is 200ms
    */
@@ -139,6 +147,7 @@ class InstantSearch extends EventEmitter {
   public _searchStalledTimer: any;
   public _isSearchStalled: boolean;
   public _searchParameters: PlainSearchParameters;
+  public _initialUiState: UiState;
   public _searchFunction?: InstantSearchOptions['searchFunction'];
   public _createURL?: (params: SearchParameters) => string;
   public _createAbsoluteURL?: (params: SearchParameters) => string;
@@ -151,7 +160,8 @@ class InstantSearch extends EventEmitter {
     const {
       indexName = null,
       numberLocale,
-      searchParameters,
+      searchParameters = {},
+      initialUiState = {},
       routing = null,
       searchFunction,
       stalledSearchDelay = 200,
@@ -234,6 +244,7 @@ See ${createDocumentationLink({
     this._stalledSearchDelay = stalledSearchDelay;
     this._searchStalledTimer = null;
     this._isSearchStalled = false;
+    this._initialUiState = initialUiState;
     this._searchParameters = {
       ...searchParameters,
       index: indexName,
@@ -411,6 +422,7 @@ See ${createDocumentationLink({
     this.mainIndex.init({
       instantSearchInstance: this,
       parent: null,
+      uiState: this._initialUiState,
     });
 
     mainHelper.search();