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

[BURG3R5]

This issue depends upon the issues #2 and #3 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.WIKI_UPDATED. If that is true, there should be two cases:

1. If len(event.wiki_pages) is equal to 1, the message should be "USER added/update the Wiki page PAGE", where USER should be event.user and PAGE should be event.wiki_pages[0].
2. Else, the message should be "USER added/updated NUMBER Wiki pages." where USER should be event.user and NUMBER should be len(event.wiki_pages).

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

This issue is a part of a bigger issue (#1). Leave a comment if you have any doubts.