Add rather dull favorite handling code

const FAVORITES_STORAGE_KEY = "songAppFavorites"

let favoriteIds = null

export function getFavoriteIds() {

    if (favoriteIds === null) {

        favoriteIds = new Set(

            JSON.parse(localStorage.getItem(FAVORITES_STORAGE_KEY)) || [],

        )

    }

    return favoriteIds

}

function setFavoriteIds(favs) {

    localStorage.setItem(

        FAVORITES_STORAGE_KEY,

        JSON.stringify(Array.from(favs)),

    )

}

export function addFavorite(id) {

    const favs = getFavoriteIds()

    favs.add(id)

    setFavoriteIds(favs)

}

export function removeFavorite(id) {

    const favs = getFavoriteIds()

    favs.delete(id)

    setFavoriteIds(favs)

}