fix(types): remove 'this' binding expectations on hook fn types (#736)

* feat(types): set void as this type of hook fns

This change improves the experience for TypeScript users that have the
[`unbound-method`](https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/unbound-method.md)
rule enabled. Without this change, check failures occur when users
attempt to use destructured references to the functions returned in the
`useAsyncStorage` hook's envelope object.

Example:

```typescript
const { getItem, setItem } = useAsyncStorage('someKey')

// eslint@typescript-eslint/unbound-method
//
// Avoid referencing unbound methods which may cause unintentional
// scoping of `this`. If your function does not access `this`, you can
// annotate it with `this: void`, or consider using an arrow function
// instead.
```

* chore: define hook types with arrow fns

This approach achieves the same end result as explicitly declaring the `this` type to be void and is more familiar to readers who know JS.

Co-authored-by: Tommy Nguyen <4123478+tido64@users.noreply.github.com>

types/index.d.ts

@@ -60,10 +60,10 @@ interface AsyncStorage {
 }
 
 type AsyncStorageHook = {
-  getItem(callback?: (error?: Error, result?: string) => void): Promise<string | null>;
-  setItem(value: string, callback?: (error?: Error) => void): Promise<void>;
-  mergeItem(value: string, callback?: (error?: Error) => void): Promise<void>;
-  removeItem(callback?: (error?: Error) => void): Promise<void>;
+  getItem: (callback?: (error?: Error, result?: string) => void) => Promise<string | null>;
+  setItem: (value: string, callback?: (error?: Error) => void) => Promise<void>;
+  mergeItem: (value: string, callback?: (error?: Error) => void) => Promise<void>;
+  removeItem: (callback?: (error?: Error) => void) => Promise<void>;
 }
 
 declare module '@react-native-async-storage/async-storage' {