use wasm_bindgen instead of handwritten js bindings

[lcnr]

I rewrote your crate to use wasm_bindgen and web_sys instead of manually written java script functions. i.e

use wasm_bindgen::prelude::*;

fn print(buf: &str) -> io::Result<()> {
    web_sys::console::info_1(&JsValue::from(buf));
    Ok(())
}

fn eprint(buf: &str) -> io::Result<()> {
    web_sys::console::warn_1(&JsValue::from(buf));
    Ok(())
}

If you want, I can polish my implementation and open a pull request.