Call init() in module_name.js for web target

[sk8ersteve]

I think it would be simpler to call "await init()" and export wasm.memory from out_dir/module_name.js instead of exporting this init function and leaving it to the user to call init() for access to memory from their js.

Current:

Generated js footer:

export { initSync }
export default init;

Code to use it:

import init, { Universe } from './out_dir/module_name.js'
let wasm = await init();
let memory = wasm.memory;

Proposed:

Generated js footer:

await init()
export let memory = wasm.memory

Code to use it:

import { Universe, memory } from './out_dir/module_name.js'

It's also a possibility to move the current generated js to the _bg.js file and put these 3 new proposed lines in its own js file similar to what is done for the nodejs target. Is there any reason we wouldn't prefer this approach? I have already made the necessary changes to wasm-bindgen in a local clone of the repo and tested it. I'm just looking for any feedback on if this is a good change or not before I go forward and create a pull request.