Add custom-set exercise

[ageron]

I set the difficulty to 3 because almost all functions are trivial except fromList which must get rid of duplicates. Perhaps it should be difficulty 4, please let me know what you think.

Also note that I focused the solution on simplicity, not performance. I considered implementing the list of items as a hash table or a sorted binary tree, but I decided that the objective of the Example.roc code is to ensure that the tests work well in the simplest way possible.

Also, the stub contains this code:

CustomSet := {
    # TODO: change this opaque type however you need
    todo : U64,
    todo2 : U64,
    todo3 : U64,
    # etc.
}
    implements [Eq]

The tests initially used == to compare CustomSet values, but this actually did not call isEq, so the tests would fail if the items inside the set were not in the same order. This is due to roc-lang/roc#7111. So I temporarily replaced == with isEq in the tests, but I left implements [Eq] in the stub and solution, so that we can easily change the tests back to == when the issue is fixed.

[isaacvando]

Awesome! I would put this as a 4 because it uses opaque types and might not be as straightforward for users who are newer to programming and haven't used sets as much.