Ask user if they want to overwrite

[nikoladev]

Right now it overwrites automatically.

Possible implementation:

1. Loop through all names in the selection and the output folder and check there are no duplicates
2. If there are no duplicates, continue as normal
3. If there ARE duplicates, ask in a dialog if they want to overwrite or abort
4. Set overwrite to true or false depending on their answer
5. Possible: show which files were skipped if they didn't want to overwrite? (show this in a scrollable list in case there is lots to show)

Dialog samples that can serve as inspiration: https://github.com/AdobeXD/plugin-samples/tree/master/ui-dialog-variations