Complete a basic example in DDD

[adenhertog]

Examples are a great way to help people learn a framework. A start of a DDD based example has been created in packages/ddd-example that models a security system. This was selected because it uses real-world concepts that are easy to grasp, and easy to communicate the code layout aspects of DDD.

This is a rather big "epic", so rather than doing everything in a single PR, this task would just be to implement the AlarmSystem agg root and some of its interactions.