Use localhost instead of 127.0.0.1 for loopback hostname

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 cloudflare/workers-sdk#3515

packages/miniflare/src/index.ts

@@ -668,16 +668,16 @@ 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