Resource / Loader Store

## Feature Request

Currently there isn't a clean way to async fetch data into a `Store`. It would be convenient if there were counterparts to `Resource` and `Loader` that use `Store`s internally rather than `Signal`s.

## Implement Suggestion

Possibly a new type `LoaderStore`, with `use_loader_store`. `LoaderStore` would be almost identical to `Loader` except internally `real_value` is a `Store>` and `read_value` is a `ReadStore`.

```rs
#[derive(Store)]
struct UserData {
name: String,
}

#[component]
fn List() -> Element {
let users = use_loader_store(move || api::get_user_data())?;

rsx! {
button { onclick: move |_| users.restart(),
"Refresh"
}
for user in users.iter() {
Row { user }
}
}
}

#[component]
fn Row(user: ReadStore) -> Element {
rsx! {
li { "{user.name()}" }
}
}
```

Start by reading the existing Resource and Loader implementations, then compare their Signal-based behavior with the Store and ReadStore APIs. The work is done when a LoaderStore and use_loader_store provide async fetching into a Store, support restart, and expose store-backed values for iteration as shown in the example.

Written by the indexing model from the issue text.