Add a page on pattern matching

[wilzbach]

e.g. based on this nice article

https://dlang.org/blog/2018/03/29/std-variant-is-everything-cool-about-d/

[PetarKirov]

I have a piece of code that could be a nice example (parsing + sumtype), although perhaps a bit advanced for beginners, so I'll assign this issue to me, as reminder.