Replace deprecated String.prototype.substr()

String.prototype.substr() is deprecated (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr) so we replace it with slice() which works similarily but isn't deprecated.
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

src/ref-helper.ts

@@ -273,8 +273,8 @@ function select(obj: any, path: string): any {
     return obj[path]
   }
 
-  const key = path.substr(0, i)
-  return select(obj[key], path.substr(i + 1))
+  const key = path.slice(0, i)
+  return select(obj[key], path.slice(i + 1))
 }
 
 function isGhes(): boolean {