Web: async improvements (#3805)

- Internal: Fix dropping `Notifier` without sending a result causing `Future`s to never complete. This should never happen anyway, but now we get a panic instead of nothing if we hit a bug.
- Internal: Remove a bunch of `unwrap()`s that aren't required when correctly using `MainThreadMarker`.
- `Window::canvas()` is now able to return a reference instead of an owned value.

Extracted from #3801.

src/platform_impl/web/event_loop/proxy.rs

@@ -1,15 +1,13 @@
-use std::rc::Weak;
-
-use super::runner::Execution;
+use super::runner::WeakShared;
 use crate::platform_impl::platform::r#async::Waker;
 
 #[derive(Clone)]
 pub struct EventLoopProxy {
-    runner: Waker<Weak<Execution>>,
+    runner: Waker<WeakShared>,
 }
 
 impl EventLoopProxy {
-    pub fn new(runner: Waker<Weak<Execution>>) -> Self {
+    pub fn new(runner: Waker<WeakShared>) -> Self {
         Self { runner }
     }