Make useOnMountEffect accept functions with void return types (#2089)

## Summary:
Previously, `useOnMountEffect` accepted functions that either return a cleanup function or explicitly return undefined. This means that functions with no return value (e.g. `() => {}`) result in errors, even though they're accepted by `useEffect`. This PR updates the argument type so that it accepts functions with void returns. Minor version bump because the API changed, but it is non-breaking.

## Test plan:
N/A

Author: timmcca-be

Reviewers: kevinbarabash

Required Reviewers:

Approved By: kevinbarabash

Checks: ✅ codecov/project, ✅ Chromatic - Get results on regular PRs (ubuntu-latest, 16.x), ✅ Test (ubuntu-latest, 16.x, 2/2), ✅ Check build sizes (ubuntu-latest, 16.x), ✅ Lint (ubuntu-latest, 16.x), ✅ Test (ubuntu-latest, 16.x, 1/2), ✅ Chromatic - Build on regular PRs / chromatic (ubuntu-latest, 16.x), ✅ Publish npm snapshot (ubuntu-latest, 16.x), ⏭  Chromatic - Skip on Release PR (changesets), ✅ Prime node_modules cache for primary configuration (ubuntu-latest, 16.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 16.x), ⏭  dependabot, ✅ gerald

Pull Request URL: #2089

.changeset/beige-kings-train.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/wonder-blocks-core": minor
+---
+
+Make useOnMountEffect accept functions with void return types

packages/wonder-blocks-core/src/hooks/use-on-mount-effect.ts

@@ -28,9 +28,7 @@ import * as React from "react";
  *
  * If you only need to do something on mount, don't return a cleanup function from `callback`.
  */
-export const useOnMountEffect = (
-    callback: () => undefined | (() => void),
-): void => {
+export const useOnMountEffect = (callback: () => void | (() => void)): void => {
     // eslint-disable-next-line react-hooks/exhaustive-deps
     React.useEffect(callback, []);
 };