Add type hint

Author: HLeithner

src/File.php

@@ -381,7 +381,7 @@ public static function invalidateFileCache($file)
      * @return  boolean  True if path is a file
      *
      */
-    public static function exists($file): bool
+    public static function exists(string $file): bool
     {
         return is_file(Path::clean($file));
     }

src/Folder.php

@@ -555,7 +555,7 @@ public static function makeSafe($path)
      *
      * @return  boolean  True if path is a folder
      */
-    public static function exists($path): bool
+    public static function exists(string $path): bool
     {
         return is_dir(Path::clean($path));
     }