feat: typed storage interface

[wzc520pyfm]

Refer: #508

We can improve the ts type as follows:

type TestObjType = {
  a: number;
  b: boolean;
};
type MyStorage = {
  foo: string;
  baz: TestObjType;
};
const storage = createStorage<MyStorage>();
await storage.getItem("foo"); // => <string>
await storage.getItem("baz"); // => <TestObjType>
storage.setItem("foo", "val"); // Check ok
storage.setItem("foo", 123); // TS error

and get hints as you type:

[pi0]

For future expansion of first generic, i prefer that we namespace it like Storage<{ data: { /* KV types */ } }>

(also can you please add ts example to a new section in the end of page? 🙏🏼

[wzc520pyfm]

sorry, I don't understand, could you explain it more detail? I guess you mean to adjust the definition of getItem like:

type NsObject = {
  data: object
  [key: string]: any
}
export interface Storage<T extends StorageValue = StorageValue> {
  // ...
  getItem<U extends Extract<T, NsObject>, K extends keyof U["data"]>(
     key: K, ops?: TransactionOptions
  ): Promise<U["data"][K] | null>;
  getItem<U extends T>(key: string, opts?: TransactionOptions ): Promise<U | null>;
}