chore(dev-middleware): add localhost as default host in start command config (facebook#44244)

Summary:
Inside [Re.Pack](https://github.com/callstack/repack) we consume command's options, to reduce the amount of assumptions that 3rd party tools need to make - we can move assigning default value to config command level, so default values will be aligned across tools.

For default `start` command this change doesn't change any behaviour.

## Changelog:

[INTERNAL] [CHANGED] - Add `localhost` as default host in `start` command config

Pull Request resolved: facebook#44244

Test Plan: `start` command should work the same way as before.

Reviewed By: huntie

Differential Revision: D56567793

Pulled By: blakef

fbshipit-source-id: fe8f3686ae39a3d2996de11930a0d03364692adc

Author: szymonrybczak

packages/community-cli-plugin/src/commands/start/index.js

@@ -27,7 +27,7 @@ const startCommand: Command = {
     },
     {
       name: '--host <string>',
-      default: '',
+      default: 'localhost',
     },
     {
       name: '--projectRoot <path>',

packages/community-cli-plugin/src/commands/start/runServer.js

@@ -34,7 +34,7 @@ export type StartCommandArgs = {
   cert?: string,
   customLogReporterPath?: string,
   experimentalDebugger: boolean,
-  host?: string,
+  host: string,
   https?: boolean,
   maxWorkers?: number,
   key?: string,
@@ -63,14 +63,13 @@ async function runServer(
     projectRoot: args.projectRoot,
     sourceExts: args.sourceExts,
   });
-  const hostname = args.host?.length ? args.host : 'localhost';
   const {
     projectRoot,
     server: {port},
     watchFolders,
   } = metroConfig;
   const protocol = args.https === true ? 'https' : 'http';
-  const devServerUrl = url.format({protocol, hostname, port});
+  const devServerUrl = url.format({protocol, hostname: args.host, port});
 
   logger.info(`Welcome to React Native v${ctx.reactNativeVersion}`);
 
@@ -104,7 +103,7 @@ async function runServer(
     messageSocketEndpoint,
     eventsSocketEndpoint,
   } = createDevServerMiddleware({
-    host: hostname,
+    host: args.host,
     port,
     watchFolders,
   });