[Swift 6]: Update window system exercise

[meatball133]

resolves #746

This removes the old closure task of the exercise, as far as I can tell is it deemed unsafe in Swift 6, and there has been several complaints about that task anyway. It is being replaced by a new initializer task instead (which is likely quite relevant to classes and struct).

TODO:

- Add proper hints file

• Update concept files to include info about initializer.

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