Enable @typescript-eslint/explicit-function-return-type in /src (#9788)

* Enable `@typescript-eslint/explicit-member-accessibility` on /src

* Prettier

* Enable `@typescript-eslint/explicit-function-return-type` in /src

* Fix types

* tsc strict fixes

* Delint

* Fix test

* Fix bad merge

src/SlidingSyncManager.ts

@@ -302,7 +302,7 @@ export class SlidingSyncManager {
      * @param batchSize The number of rooms to return in each request.
      * @param gapBetweenRequestsMs The number of milliseconds to wait between requests.
      */
-    public async startSpidering(batchSize: number, gapBetweenRequestsMs: number) {
+    public async startSpidering(batchSize: number, gapBetweenRequestsMs: number): Promise<void> {
         await sleep(gapBetweenRequestsMs); // wait a bit as this is called on first render so let's let things load
         const listIndex = this.getOrAllocateListIndex(SlidingSyncManager.ListSearch);
         let startIndex = batchSize;