atomWithStorage subscribe doesn't allow undefined as a return type

[caffeinewriter]

Bug Description

When providing a custom subscribe method, the return typing of Subscribe does not allow the function to simply return undefined. This is contrary to the example given in the docs for using a validator in atomWithStorage.

https://jotai.org/docs/utilities/storage#validating-stored-values

In addition, while undefined makes sense to return when there's no cleanup to do, this example in the documentation should probably return a cleanup function to remove the storage event listener.

e.g.

import { atomWithStorage } from 'jotai/utils'
import { z } from 'zod'

const myNumberSchema = z.number().int().nonnegative()

const storedNumberAtom = atomWithStorage('my-number', 0, {
  getItem(key, initialValue) {
    const storedValue = localStorage.getItem(key)
    try {
      return myNumberSchema.parse(JSON.parse(storedValue ?? ''))
    } catch {
      return initialValue
    }
  },
  setItem(key, value) {
    localStorage.setItem(key, JSON.stringify(value))
  },
  removeItem(key) {
    localStorage.removeItem(key)
  },
  subscribe(key, callback, initialValue) {
    if (
      typeof window === 'undefined' ||
      typeof window.addEventListener === 'undefined'
    ) {
      return
    }
    const handler = (e: StorageEvent) => {
      if (e.storageArea === localStorage && e.key === key) {
        let newValue
        try {
          newValue = myNumberSchema.parse(JSON.parse(e.newValue ?? ''))
        } catch {
          newValue = initialValue
        }
        callback(newValue)
      }
    }
    window.addEventListener('storage', handler)
    return () => window.removeEventListener('storage', handler);
  },
})

Reproduction Link

https://stackblitz.com/edit/vitejs-vite-8v2agps7

[caffeinewriter]

I've created PRs to address these separately.

#3055 to fix the return typing
#3056 to update the docs to return a cleanup function for the storage subscriber