Extend ReactiveScope into scopes that are siblings

[lukechu10]

This PR adds a new ReactiveScope::extend method which allows running a function in the ReactiveScope after it has already been created. This is especially useful together with wasm_bindgen_futures::spawn_local to extend the scope into inside the async block.

This also adds spawn_local_in_scope which is a simple wrapper around wasm_bindgen_futures::spawn_local but which extends the scope into inside the async block.

Documentation about preloading data has been updated to use this.

Fixes #276

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