feat: allow line-wrapping when printing DiscrTree.Keys (#7200)

This PR allows the debug form of DiscrTree.Key to line-wrap.

src/Lean/Meta/DiscrTree.lean

@@ -96,13 +96,14 @@ where
     if args.isEmpty then
       return f
     else
-      let mut r := f
+      let mut r := m!""
       for arg in args do
-        r := r ++ m!" {arg}"
+        r := r ++ Format.line ++ arg
+      r := f ++ .nest 2 r
       if parenIfNonAtomic then
-        return m!"({r})"
+        return .paren r
       else
-        return r
+        return .group r
 
   go (parenIfNonAtomic := true) : StateRefT (List Key) CoreM MessageData := do
     let some key ← next? | return .nil