Extract the confirm call in its own, overridable method in rails_ujs

[mathieumahe]

Following the discussion in #32402 and the inactivity of #29965, I added the possibility to override the confirm call in rails_ujs. This feature can be useful to anyone who want to customize the confirm dialog for example.

The behaviour was possible with jquery_ujs but was missing from rails_ujs.

[rails-bot]

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[orischwartz]

Any reason why this commit (91f4e33) has not been pulled into https://github.com/rails/rails/tree/5-2-stable?

There's no risk of breaking any existing functionality and it's a useful enhancement for the rails-ujs npm package.

[brendon]

This would have been so good to get into Rails 5.2. Is there any chance of that happening now? Rails 6 wasn't out when this one was merged.