Use localhost instead of 127.0.0.1 for loopback hostname (#615)

Miniflare starts its loopback server on the same host as the
`workerd` server, so it's accessible on all the same addresses as the
runtime. This allows the loopback server to be the target for the
live reload web socket. This means if the `host: "localhost"` option
is set, we start a Node `http` server with `localhost` as the
hostname. Node will perform a DNS lookup to work out which IP address
and interface to listen on, but will only listen on the first entry.
In Node 17+, this will be the IPv6 interface. Unfortunately, we
previously hardcoded the loopback address as the IPv4 loopback,
meaning `workerd` was unable to connect. This change switches to
using `localhost`, which `workerd` will resolve to either IPv4 or v6.

Closes #3515

packages/miniflare/src/index.ts

@@ -733,16 +733,16 @@ export class Miniflare {
 	};
 
 	#startLoopbackServer(
-		port: string | number,
-		hostname?: string
+		port: number,
+		hostname: string
 	): Promise<StoppableServer> {
 		return new Promise((resolve) => {
 			const server = stoppable(
 				http.createServer(this.#handleLoopback),
 				/* grace */ 0
 			);
 			server.on("upgrade", this.#handleLoopbackUpgrade);
-			server.listen(port as any, hostname, () => resolve(server));
+			server.listen(port, hostname, () => resolve(server));
 		});
 	}
 

packages/miniflare/src/runtime/index.ts

@@ -87,7 +87,7 @@ export class Runtime {
 			// (e.g. "streams_enable_constructors"), see https://github.com/cloudflare/workerd/pull/21
 			"--experimental",
 			`--socket-addr=${SOCKET_ENTRY}=${this.opts.entryHost}:${this.opts.entryPort}`,
-			`--external-addr=${SERVICE_LOOPBACK}=127.0.0.1:${this.opts.loopbackPort}`,
+			`--external-addr=${SERVICE_LOOPBACK}=localhost:${this.opts.loopbackPort}`,
 			// Configure extra pipe for receiving control messages (e.g. when ready)
 			"--control-fd=3",
 			// Read config from stdin