Fix wasm

[tychedelia]

Because we weren't creating a default window in WindowPlugin, the renderer wasn't able to initialize with the correct surface to attach to the canvas. This PR fixes this by doing some hacks when compiling under wasm32 to create the default window and then update it to whatever the user provides in their WindowBuilder. This only supports a single primary window on web.

Ideally, long term, we'd figure out a better pattern for spawning a new canvas element when the window is created dynamically, rather than relying on Bevy's window initialization logic in the renderer. I'm sure this is possible, just need some more consultation with some other Bevy folks to figure out how to do it.

Test with:

cargo build --release --example draw --target wasm32-unknown-unknown
wasm-bindgen --out-name draw \ 
    --out-dir examples/wasm/target \
    --target web target/wasm32-unknown-unknown/release/examples/draw.wasm
basic-http-server examples/wasm