fix: add shebangs to async SDK implementations

Add #!/usr/bin/env interpreter lines to async SDKs for portability:
- Python: #!/usr/bin/env python3
- JavaScript: #!/usr/bin/env node
- Ruby: #!/usr/bin/env ruby
- PHP: #!/usr/bin/env php

clients/javascript/async/src/un_async.js

@@ -1,3 +1,4 @@
+#!/usr/bin/env node
 /**
  * PUBLIC DOMAIN - NO LICENSE, NO WARRANTY
  *

clients/php/async/src/UnsandboxAsync.php

@@ -1,3 +1,4 @@
+#!/usr/bin/env php
 <?php
 /**
  * PUBLIC DOMAIN - NO LICENSE, NO WARRANTY

clients/python/async/src/un_async.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python3
 """
 PUBLIC DOMAIN - NO LICENSE, NO WARRANTY
 

clients/ruby/async/src/un_async.rb

@@ -1,3 +1,4 @@
+#!/usr/bin/env ruby
 # frozen_string_literal: true
 
 # PUBLIC DOMAIN - NO LICENSE, NO WARRANTY