Ability handler counter concept

[rlmark]

First try at adding a concept with first concept exercise.

This concept is about writing your own algebraic effect handlers (called abilities) in Unison. It will eventually have a prerequisite, which is about using abilities, but ability handlers are a huge missing piece for our existing community in terms of exercises and practice opportunities. A branch for the "using pre-made abilities and ability handlers" content can be found here.

The idea with the planned ability-handlers concept exercises is that we'll start with simple ability handlers (here we just model a Counter effect) and guide learners towards writing more complicated handlers for different abilities.