feat: generic type support

[maou-shonen]

It would be nice to have generics support,
This can enhance DX.

some examples

import { createStorage } from "./storage";
import { prefixStorage } from "./utils";

// If no type is specified, the behavior remains the same as before
const baseStorage = createStorage(); // Storage<StorageValue>
baseStorage.getItem("k"); // => Promise<StorageValue>
baseStorage.getItemRaw("k"); // => Promise<any>
baseStorage.setItem("k", 1); // value allow StorageValue
baseStorage.setItemRaw("k", 1); // value allow any

// If type is specified
baseStorage.getItem<number>("k"); // => Promise<number>
baseStorage.getItemRaw<number>("k"); // => Promise<number>
baseStorage.setItem<number>("k", 1); // value allow number
baseStorage.setItemRaw<number>("k", 1); // value allow number

// forward reference
const numberStorage = createStorage<number>(); // Storage<number>
numberStorage.getItem("k"); // => Promise<number>
numberStorage.getItemRaw("k"); // => Promise<number>
numberStorage.setItem("k", 1); // => Promise<number>
numberStorage.setItemRaw("k", 1); // => Promise<number>

// overriding incompatible types is not allowed
numberStorage.getItem<string>("k", "1"); // ts error
numberStorage.setItem<string>("k", "1"); // ts error

// custom type
type Foo = {
  a: number;
};
const customTypeStorage = createStorage<Foo>();
customTypeStorage.getItem("k"); // Promise<Foo>
customTypeStorage.setItem("k", { a: 123 }); // value allow Foo

// prefixStorage
prefixStorage(baseStorage, "assets").getItem("k"); // Promise<StorageValue>
prefixStorage<number>(baseStorage, "numbers").getItem("k"); // Promise<number>

Regarding the inability to override,
perhaps it is overly restrictive? What do you think?

// overriding incompatible types is not allowed
numberStorage.getItem<string>("k", "1"); // ts error

Also, my English is weak,
if there are any inaccuracies in the documentation,
anyone is welcome to modify it.

[nuxt-studio]

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Name	Edit	Preview	Latest Commit
unstorage	Edit on Studio ↗︎	View Live Preview	fa03a14

[codecov]

Codecov Report

Merging #237 (fa03a14) into main (7520633) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   80.76%   80.85%   +0.09%     
==========================================
  Files          24       24              
  Lines        2682     2695      +13     
  Branches      446      445       -1     
==========================================
+ Hits         2166     2179      +13     
  Misses        515      515              
  Partials        1        1              

Impacted Files	Coverage Δ
src/storage.ts	88.03% <100.00%> (+0.06%)	⬆️
src/types.ts	100.00% <100.00%> (ø)
src/utils.ts	93.65% <100.00%> (+0.31%)	⬆️