Support defining data schema for storage

[jalooc]

While the ability to define the storage data type when using functions like storage.defineItem<Type>() is convenient, it's sadly not really type-safe - the contents of the storage can be modified either by code not using the same storage util, or even manually, so it's not guaranteed that we always get data of type Type when using getValue().

The idea is to allow passing a schema (preferably a zod schema) against which the value read from the storage would be validated internally in getValue() before being returned. A nice bonus is that it would be easy to automatically infer the type based on the schema, so the dev wouldn't have to specify it on top of the schema.

It could look sth like this:

const myStorage = storage.defineItem('local:my-storage', z.object({ foo: z.number() }))

myStorage.getValue().then(value => /* fully type-safe `{ foo: number }` */)

and the types for defineItem() would go sth like this:

import { z } from 'zod'

const defineItem = <Schema extends z.ZodType<unknown>>(key: string, schema: Schema) => {
  getValue: () => z.infer<Schema>,
}

Alternatively, if we wanted to avoid vendor-lockin on zod, we could just have a validate parameter:

const defineItem = <T>(key: string, validate: () => T) => {
  getValue: () => T,
}