Wingman destruct doesn't split case matches

[isovector]

test :: Maybe Bool -> a
test x = case x of
  Just y -> _

case split on y here doesn't produce

test :: Maybe Bool -> a
test x = case x of
  Just True -> _
  Just False -> _

but it should!

[isovector]

I was looking into fixing this today, but it's subtly challenging. The issue is that Wingman is good at grafting holes, but here we want to graft over the case that contains the hole --- and that's not an easy thing to get our hands on. It works for top-level holes because we can just get the smallest containing MatchGroup which is guaranteed to be the immediate parent, but in the HsExpr case, the nearest MatchGroup might be a containing case statement that is nowhere near the hole.

WIP branch:
https://github.com/haskell/haskell-language-server/compare/master...isovector:agda-split-more-often?expand=1