support building binary crates

[boringcactus]

💡 Feature description

Using wasm-pack to build libraries is nice, but binaries are also good to have, both in the npm ecosystem and for browser usage (see also wasm-bindgen/wasm-bindgen#1630 and rust-gamedev/wg#51). However, wasm-pack asserts in a few places that all crates being built are libraries.

💻 Basic example

A crate with the following code in src/main.rs should build and run as intended with wasm-pack build --target web:

#[cfg(target_arch = "wasm32")]
use wasm_bindgen::prelude::*;

#[cfg(target_arch = "wasm32")]
fn display(text: &str) {
    // Use `web_sys`'s global `window` function to get a handle on the global
    // window object.
    let window = web_sys::window().expect("no global `window` exists");
    let document = window.document().expect("should have a document on window");
    let body = document.body().expect("document should have a body");

    // Manufacture the element we're gonna append
    let val = document.create_element("p").expect("failed to create a <p>");
    val.set_inner_html(text);

    body.append_child(&val).expect("failed to append child to body");
}

#[cfg(not(target_arch = "wasm32"))]
fn display(text: &str) {
    println!("{}", text);
}

fn main() {
    display("Hello from Rust!");
}

(I've made the changes needed in wasm-bindgen already; they're included in v0.2.54. I have a basic implementation for this feature already, but it needs additional tuning to behave as expected in all cases.)