Implement DAP 'terminated' event

This implements the DAP 'terminated' event.  Vladimir Makaev noticed
that VSCode will not report the debug session as over unless this is
sent.

It's not completely clear when exactly this event ought to be sent.
Here I've done it when the inferior exits.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30681

gdb/python/lib/gdb/dap/events.py

@@ -32,6 +32,7 @@ def _on_exit(event):
             "exitCode": code,
         },
     )
+    send_event("terminated")
 
 
 @in_gdb_thread

gdb/python/lib/gdb/dap/server.py

@@ -151,7 +151,7 @@ class Server:
         self.done = True
 
 
-def send_event(event, body):
+def send_event(event, body=None):
     """Send an event to the DAP client.
     EVENT is the name of the event, a string.
     BODY is the body of the event, an arbitrary object."""