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/hooks/useTimeout.ts

@@ -19,7 +19,7 @@ import { useEffect, useRef, useState } from "react";
 type Handler = () => void;
 
 // Hook to simplify timeouts in functional components
-export const useTimeout = (handler: Handler, timeoutMs: number) => {
+export const useTimeout = (handler: Handler, timeoutMs: number): void => {
     // Create a ref that stores handler
     const savedHandler = useRef<Handler>();
 
@@ -38,7 +38,7 @@ export const useTimeout = (handler: Handler, timeoutMs: number) => {
 };
 
 // Hook to simplify intervals in functional components
-export const useInterval = (handler: Handler, intervalMs: number) => {
+export const useInterval = (handler: Handler, intervalMs: number): void => {
     // Create a ref that stores handler
     const savedHandler = useRef<Handler>();
 
@@ -57,7 +57,7 @@ export const useInterval = (handler: Handler, intervalMs: number) => {
 };
 
 // Hook to simplify a variable counting down to 0, handler called when it reached 0
-export const useExpiringCounter = (handler: Handler, intervalMs: number, initialCount: number) => {
+export const useExpiringCounter = (handler: Handler, intervalMs: number, initialCount: number): number => {
     const [count, setCount] = useState(initialCount);
     useInterval(() => setCount((c) => c - 1), intervalMs);
     if (count === 0) {