[lib] Add containedThreadInfos selector

Summary:
Part of [ENG-4319](https://linear.app/comm/issue/ENG-4319/warn-the-user-when-deleting-a-chat-that-has-contained-chats).

A new selector is needed to obtain contained chats. There exists `childThreadInfos` selector, but it operates on `parentThreadID` rather than on `containingThreadID`.

Depends on D8686.

Test Plan: Tested later in the stack.

Reviewers: bartek, tomek, inka, michal, ashoat

Reviewed By: bartek, inka, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8526

lib/selectors/thread-selectors.js

@@ -200,6 +200,27 @@ const childThreadInfos: (state: BaseAppState<>) => {
   },
 );
 
+const containedThreadInfos: (state: BaseAppState<>) => {
+  +[id: string]: $ReadOnlyArray<ThreadInfo>,
+} = createSelector(
+  threadInfoSelector,
+  (threadInfos: { +[id: string]: ThreadInfo }) => {
+    const result = {};
+    for (const id in threadInfos) {
+      const threadInfo = threadInfos[id];
+      const { containingThreadID } = threadInfo;
+      if (containingThreadID === null || containingThreadID === undefined) {
+        continue;
+      }
+      if (result[containingThreadID] === undefined) {
+        result[containingThreadID] = [];
+      }
+      result[containingThreadID].push(threadInfo);
+    }
+    return result;
+  },
+);
+
 function getMostRecentRawMessageInfo(
   threadInfo: ThreadInfo,
   messageStore: MessageStore,
@@ -451,6 +472,7 @@ export {
   entryInfoSelector,
   currentDaysToEntries,
   childThreadInfos,
+  containedThreadInfos,
   unreadCount,
   unreadBackgroundCount,
   otherUsersButNoOtherAdmins,