Add new `elif` clause to `compose_message` for `LABEL_UPDATED` event.

[BURG3R5]

This issue depends upon the issues #7 and #8 and thus can be only attempted after those issues are closed.

In this issue, you'll add an elif clause to the compose_message function of the Messenger class present in the file bot/slack/messenger.py

The elif clause will check if the event.type is equal to EventType.LABEL_UPDATED. If that is true, you should set message to "USER ACTION the label LABEL", where USER should be event.user, ACTION should be event.label.action and LABEL should be event.label.

You should use f-strings as seen in other clauses already present in the function.

This issue is a part of a bigger issue (#6). Leave a comment if you have any doubts.