Add the langPath() function to the Application Contract (#40932)

To help with type hinting

Author: wilsenhc

src/Illuminate/Contracts/Foundation/Application.php

@@ -45,6 +45,14 @@ public function configPath($path = '');
      */
     public function databasePath($path = '');
 
+    /**
+     * Get the path to the language files.
+     *
+     * @param  string  $path
+     * @return string
+     */
+    public function langPath($path = '');
+
     /**
      * Get the path to the resources directory.
      *