feat(core): add WindowEvent::FileDrop, closes #3664

[lucasfernog]

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Does this PR introduce a breaking change?

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[FabianLars]

With that change we could theoretically remove the tauri:// events from rust again right? They are not reallyyy usable anyway (and afaik pretty much nobody uses them either) 🤷

Of course we still need the updater events.

[lucasfernog]

Well we can also offer the updater events as part of the App::run callback. I think we'll rely on it more for v2.

[lucasfernog]

@FabianLars #3687 crazy change but at least no one has to deal with the magic string event payloads on the Rust side.