Add zipper exercise

[ageron]

Adds the Zipper exercise.

Note: The solution (.meta/Example.roc) contains a really ugly up function. I couldn't find a better way to implement it because it doesn't seem possible to easily add a default field to a record (see this Zulip discussion: #beginners > Cannot add fields to a record with default fields). That said, this is just an example solution, the stub (Tree.roc) remains reasonably elegant.

Note 2: I pondered whether I should provide the definition of the Zipper type. I chose to leave it out because thinking about how you might design a zipper is half the fun. My original solution was horrible, so I eventually cheated and asked an AI how zippers are typically implemented for trees. Coming up with something and then learning about something better was a much better learning experience than simply being handed the best design from the start. Once you know the design, implementing it isn't too difficult.

Note 3: in my solution, the Crumb type is currently:

	Crumb : [
		Left({ value : U64, right : Try(Tree, [MissingField]) }),
		Right({ value : U64, left : Try(Tree, [MissingField]) }),
	]

Once the discussion about adding default fields to records is resolved, it will probably be better to implement it as:

	Crumb : [
		Left({ value : U64, right ?: Tree }),
		Right({ value : U64, left ?: Tree }),
	]

[kotp]

I eventually cheated and asked

This is not cheating, since Exercism is not a testing platform. This is a learning platform, and asking for help is definitely not cheating.

[Anton-4]

Is this ready to merge @ageron? I am mainly wondering if it's feasible for exercism students to find the elegant solution or if it will mostly be frustrating :p

[ageron]

Thanks for reviewing, @anton. Yes, I believe it's ready to merge: it's difficult but doable, and we don't have that many difficult exercises, so I think it's ok.

[Anton-4]

Thanks @ageron!