api: list information about all channels/jobs.

Fire autocmd when channel opens or its info changes.
Add a way for API clients can describe themselves.

src/nvim/os/pty_process_win.c

@@ -183,6 +183,11 @@ cleanup:
   return status;
 }
 
+const char *pty_process_tty_name(PtyProcess *ptyproc)
+{
+  return "?";
+}
+
 void pty_process_resize(PtyProcess *ptyproc, uint16_t width,
                         uint16_t height)
   FUNC_ATTR_NONNULL_ALL