Fix HelloWorld Samples (#1505)

* Fixed HelloWorld samples

- Samples were not starting up without a main function

* Fixed code review comments

Author: Sam-Amin

Sources/Examples/HelloWorld/Client/main.swift renamed to Sources/Examples/HelloWorld/Client/helloworldclient.swift

@@ -20,6 +20,7 @@ import HelloWorldModel
 import NIOCore
 import NIOPosix
 
+@main
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 struct HelloWorld: AsyncParsableCommand {
   @Option(help: "The port to connect to")

Sources/Examples/HelloWorld/Server/main.swift renamed to Sources/Examples/HelloWorld/Server/HelloWorldServer.swift

@@ -20,6 +20,7 @@ import HelloWorldModel
 import NIOCore
 import NIOPosix
 
+@main
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 struct HelloWorld: AsyncParsableCommand {
   @Option(help: "The port to listen on for new connections")