Integrate async function with async init

[RReverser]

Motivation

#1905 added syntactic support for async + #[wasm_bindgen(start)], which means it's accepted by the macro, but it's not changing anything on the JS side.

It's leading to confusing race conditions where an async function marked with #[wasm_bindgen(start)] is doing some preparatory async work, and user expects it to be finished by the time await init(); is finished on --target web (or an equivalent or other targets).

On practice, however, wasm-bindgen generated init will only kickstart the async process, but not wait for its completion, so code like this fails in unexpected ways:

#[wasm_bindgen(start)]
pub async fn start() {
  // ...init some global state
}

#[wasm_bindgen]
pub fn foo() {
  // ...access the global state assuming it's ready
}

import init, { foo } from './pkg';

await init();
foo();

Proposed Solution

async functions marked with #[wasm_bindgen(start)] should create a promise that is awaited or returned from the wasm-bindgen-generated init function (or its equivalent on other targets).

Alternatives

As an alternative, async + #[wasm_bindgen(start)] could be disallowed to make it more clear that the function itself won't be awaited.

Additional Context

Add any other context or screenshots about the feature request here.