fix(messages): better formatting for ext_messages #31839

Problem:  Message grid newline formatting based on `msg_col` is not
          utilized with ext_messages.
Solution: Increment `msg_col` with the cell width of the chunk. Allowing
          message code that uses `msg_col` to determine when to place a
          newline to do so. E.g. when the message goes beyond `Columns`;
          this is not necessarily where the ext_messages implementation
          would want to place a newline, but it is a best guess. Message
          parsing and manipulation is still possible.

src/nvim/digraph.c

@@ -1714,6 +1714,7 @@ void listdigraphs(bool use_headers)
 {
   result_T previous = 0;
 
+  msg_ext_set_kind("list_cmd");
   msg_putchar('\n');
 
   const digr_T *dp = digraphdefault;