compiler: rename ZonkAny to UnusedType and add pretty printing logic

ZonkAny is a hard to understand name for users who do not know how the compiler works internally. Additionally, it is confusing that ZonkAny, while being a concrete type represents a meta variable, espeically in the compiler output. This patch changes the name of ZonkAny to UnusedType which is closer to its intended semantics and adds special pretty printing logic to display this type in the same fashion the compiler displays meta variables in other places, whenever they leak from the implementation to the user.

Changes contained in this patch

MR Checklist

• This MR solves the problem described in the following issue: #27390 (closed)
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