Storage Item Serialization and Validation

Feature Request

When defineing storage items, getting values, or setting values, we should support serialization options for converting storage to and from a specific data type:

const enabledSites = storage.defineItem<Set<string>, {}, string[]>("enabledSites", {
  schema: {
    // Convert usable value to storage compatible value
    serialize: (set) => [...set],
    // Convert storage compatible value to usable type
    deserialize: (array) => new Set(array),
  },
});

const set = await enabledSites.getValue();
set.has("google.com")

A natural extension to this is to support various other popular data serialization and validation libraries, like Zod and TypeBox.

import { z } from 'zod';

const themePreference: WxtStorageItem<"light" | "dark" | "system"> = storage.defineItem("themePreference", {
  schema: z.enum(["light", "dark", "system"]),
});

import { Type } from '@sinclair/typebox';
import { Value } from '@sinclair/typebox';

const themePreference: WxtStorageItem<"light" | "dark" | "system"> = storage.defineItem("themePreference", {
  schema: (value) => Value.Parse(
    Type.Union([Type.Literal("light"), Type.Literal("dark"), Type.Literal("system")]),
    value,
  ),
});

Is your feature request related to a bug?

https://github.com/wxt-dev/wxt/discussions/1123

What are the alternatives?

Write a wrapper around storage.defineItem yourself.

Additional context

N/A

Start by tracing the storage.defineItem entry point and its schema handling. Determine how serialize and deserialize callbacks should affect defining, reading, and setting storage items, then consider the requested Zod and TypeBox validation patterns. Done means storage items support the shown Set-to-array conversion and schema validation behavior.