add hello_world binary

Author: withered-magic

examples/deep_gem/hello_world/bin/hello_world

@@ -0,0 +1,8 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require "hello_world"
+
+name = ARGV[0] || "World"
+speaker = HelloWorld::Speaker.new(name)
+puts speaker.message

examples/deep_gem/hello_world/hello_world.gemspec

@@ -14,7 +14,8 @@ Gem::Specification.new do |s|
 
   s.summary = "Example"
   s.description = "Example gem"
-  s.files = Dir["lib/**/*"]
+  s.files = Dir["lib/**/*"] + Dir["bin/*"]
+  s.executables = ["hello_world"]
 
   s.require_paths = ["lib"]
   s.required_ruby_version = Gem::Requirement.new(">= 2.6")