feat: wasm dev server

[tchardin]

Building wasm:

• Run wasm-pack build --target web --out-dir web/src/wasm -- --features browser --no-default-features. This will make sure the right JS files are generated directly in the JS source directory.
• You will still need to move /web/src/wasm/pop_bg.wasm to /web/public/pop_bg.wasm for it to load properly as it is served separately.
• To run the web server open a terminal into /web and run npm run start then open http://localhost:8000 in an incognito window (helps register a new service worker each time otherwise you will be running the previous service worker when reloading the page and changes won't take effect).
• Copy the peer multiaddress (/p2p/) and root CID. You can run it against a pop and if everything works you can retrieve the content for that CID into memory.

Blockers:

• Currently getting an error message when calling request: Failed to create executor thread pool: Error { kind: Unsupported, message: "operation not supported on this platform" }. Not sure what is causing it, I wasn't aware of a thread pool usage anywhere.
• I haven't found a way to keep an instance of the swarm and call multiple requests on it from the service worker handler. See https://github.com/rustwasm/wasm-bindgen/issues/1858 for more context.

[alexander-camuto]

for reference should pass an executor to the swarm that is compatible with wasm. For instance:

let mut swarm = SwarmBuilder::new(transport, behaviour, local_peer_id)
            .executor(Box::new(|fut| {
                wasm_bindgen_futures::spawn_local(fut);
            }))
            .build();

[alexander-camuto]

delete

 let mut yamux_config = yamux::YamuxConfig::default();
        yamux_config.set_max_buffer_size(16 * 1024 * 1024);
        yamux_config.set_receive_window_size(16 * 1024 * 1024);
        core::upgrade::SelectUpgrade::new(yamux_config, mplex_config)

as SelectUpgrade seems to hang forever in the browser. Multiplexer should just be the default mplex::MplexConfig::new()