feat(api): set nvim_echo() kind for ext_messages (#33998)

Problem:  Unable to emit a message with arbitrary kind.
Solution: Add a "kind" opts field to nvim_echo().
          Use it to set the "list_cmd" kind for vim.show_pos().

runtime/lua/vim/_inspector.lua

@@ -270,5 +270,5 @@ function vim.show_pos(bufnr, row, col, filter)
       },
     }
   end
-  vim.api.nvim_echo(chunks, false, {})
+  vim.api.nvim_echo(chunks, false, { kind = 'list_cmd' })
 end