Add build function check for current OS and update validation logic

src/StaticPHP/Package/Package.php

@@ -127,6 +127,14 @@ abstract class Package
         return false;
     }
 
+    /**
+     * Check if the package has a build function for the current OS.
+     */
+    public function hasBuildFunctionForCurrentOS(): bool
+    {
+        return isset($this->build_functions[PHP_OS_FAMILY]);
+    }
+
     /**
      * Get the name of the package.
      */