fix: safari copy item issue

[getanwar]

For effectAllowed we can not rely on drag event properties since different browsers give different results. In Safari (MacOS) I always received effectAllowed = 'all', does not matter I assign the property (effect-allowed) to vddl-draggable or not. It always triggers move effect. So what I did here is except relying on browser's event, I made the tweak on existing prototype object (vddlDropEffectWorkaround.dropEffect) that you created for wired browser behavior. It solves my issue.

Please accept my pull request, if you don't, tell me what else we can do to solve this quirk.

[hejianxian]

@getanwar

Sorry, I am very busy recently. Thank you for your PR, I will check it as soon as possible.