pub fn fallback(mut self, view: impl Fn(Scope, P) -> View<G> + Send + Sync + 'static) -> Self {
        {
            self.fallback = Some(Arc::new(view));
        }
        self
    }
Declares the fallback view to render for this capsule. When Perseus
renders a page of your app, it fetches the page itself, along with
all the capsules it needs. If the page is ready before all the
capsules, then it will be displayed immediately, with fallback views
for the capsules that aren't ready yet. Once they are ready, they
will be updated.

This fallback view cannot access any of the state that the capsule
generated, but it can access any properties provided to it by the
page, along with a translator and the like. This view is fully
reactive, it just doesn't have the state yet.

**Warning:** if you do not set a fallback view for a capsule, your app
will not compile!
packages/perseus/src/template/capsule.rs