fix(messages): "progress" kind for busy messages #39280

Problem:  The "Scanning:" completion, bufwrite, and indent (there may be
          more) messages which indicate progress can use the "progress" kind
          for their msg_show event. Indent message does not have a kind.

Solution: Emit these messages with the "progress" kind. Set the message id
          to the replaced kind so that a UI knows to replace it (and to provide
          a migration path in case a UI was distinguishing these messages for
          whatever reason).

test/functional/ui/screen.lua

@@ -1453,6 +1453,12 @@ function Screen:_handle_msg_show(kind, chunks, replace_last, history, append, id
   if not replace_last or pos == 0 then
     pos = pos + 1
   end
+  for i, msg in pairs(self.messages) do
+    if id ~= -1 and msg.id == id then
+      pos = i
+      break
+    end
+  end
   self.messages[pos] = {
     kind = kind,
     content = chunks,