Change the default example

[jtpio]

For now the example creates a new canvas from Rust:

https://github.com/jtpio/jupyterlab-wasm-example/blob/6ddbf4f1bdc56558840e2416ec49453c2632e241/rust/src/lib.rs#L25-L29

While it is quite remarkable, it might not be the most appropriate use case since this could arguably be done more easily in JS / TS directly.

It would be interesting to switch to an example that really justifies using Rust and WebAssembly in an extension.