remap with null handling

```ts
interface Value {
foo: number | null;
bar?: string;
}
const $demo = createStore(null);

const [$foo, $bar] = remap($demo, ["foo", "bar"]);
// type: [Store, Store]

const $justFoo = remap($demo, "foo");
// type: Store
```

Alternative names?
- `rechain`

Start with the remap API and the TypeScript examples in this issue. Determine whether the requested null-aware typing and naming should be adopted. Done would require a maintainer-approved API direction and implementation scope, but the issue identifies no files or tests.

Written by the indexing model from the issue text.