Add to raise an error when executed multiple times

[taketo1113]

Summary

Raise an error to prevent multiple event listeners from being registered when alt-ujs is executed more than once.

Details

If start() is executed twice as shown below:

import AltUjs from "alt-ujs";

AltUjs.start();

...

AltUjs.start();

The following error occurs:

Uncaught Error: alt-ujs: confirm has already been loaded.

(confirm is the module name of alt-ujs that was called twice.)