Enhancement: `ls` uses `message_type` field to distinguish JSON messages

The `ls` command was the only command that used the `struct_type` field to determine
the message type in the JSON output format. Now, the JSON output of the
`ls` command also includes the `message_type`. The `struct_type` field is
still included, but it deprecated.

https://github.com/restic/restic/pull/4664
