WASM multiple canvas

In Bevy there is currently [no way of using multiple canvas elements.](https://github.com/bevyengine/bevy/issues/20453#issuecomment-3677872675)

I'm trying to render multiple canvases using this code.
It's based on the [multiple_windows](https://github.com/bevyengine/bevy/blob/main/examples/window/multiple_windows.rs) example.

When it is ran in the browser I get this error output
```
panicked at /Users/kelp/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bevy_render-0.16.0/src/view/window/mod.rs:338:51:
No supported formats for surface

Uncaught RuntimeError: unreachable
```

```html

```

```rust

fn main() {
App::new()
.add_plugins(DefaultPlugins.set(WindowPlugin {
primary_window: Some(Window {
canvas: Some("#bevy_canvas1".to_string()),
..default()
}),
..default()
}))
.add_systems(Startup, setup_scene)
.run();
}

fn setup_scene(mut commands: Commands) {

let first_window_camera = commands
.spawn(Camera2d)
.id();

// Spawn a second window
let second_window = commands
.spawn(Window {
canvas: Some("#bevy_canvas2".to_string()),
title: "Second window".to_owned(),
..default()
})
.id();

let second_window_camera = commands
.spawn((
Camera2d,
Camera {
target: RenderTarget::Window(WindowRef::Entity(second_window)),
..default()
},
))
.id();

let node = Node {
position_type: PositionType::Absolute,
top: Val::Px(12.0),
left: Val::Px(12.0),
..default()
};

commands
.spawn((
node.clone(),
UiTargetCamera(first_window_camera),
))
.with_child((Text::new("First window"), TextShadow::default()));

commands
.spawn((node, UiTargetCamera(second_window_camera)))
.with_child((Text::new("Second window"), TextShadow::default()));
}

```

Start with examples/window/multiple_windows.rs and the browser reproduction, then inspect bevy_render-0.16.0/src/view/window/mod.rs around line 338 where the surface-format panic occurs. Confirm the behavior with two HTML canvas elements; done means multiple Bevy windows render in their separate canvases without the browser panic.

Written by the indexing model from the issue text.