Fix case class default parameter detection and annotation detection in Scala3

[lihaoyi]

Fixes #33, as well as @arg annotations not working on case class arguments (which doesn't have an open issue)

Just need some special casing for apply/<init>, both for getting the apply defaults (which are named <init>$default$n since they're shared) and for getting the apply parameter annotations (which end up on the <init> method parameters, rather than the apply method)

Re-enabled a bunch of previously-disabled tests, and added a new suite to exercise default parameters in direct/nested scenarios