Add example for Javascript callbacks

[orhun]

See #81 (comment)

just something simple, maybe: a label populated by a rust function after init ("ratzilla initialized") and a text field which forwards any input to rust. if you're brave, maybe have a go a holding onto callbacks.

the tfx-ftl setup is a bit too crude for handling js callbacks on the rust side, but take a look at https://github.com/junkdog/tachyonfx-ftl/blob/main/tachyonfx-ftl-web/src/interop.rs (no additional config, apart from the wasm-bindgen dependency) .