Enable whole word only notifications by default

Set PREF_NOTIFY_MENTION_WHOLE_WORD to true.

If I'm not mistaken the _mucwin_print_mention() / get_mentions()
functions only work correctly since
6bc440c6f7.

This changed the behaviour for users.
They got notified when their nick was `kaffee` and in the message the
string `kaffeekanne` occured.

Setting `/notify room mention word_whole` corrected this.

So my idea is that only now the mention function work correctly. And to
have a good default behaviour we should set the `word_whole` on by
default.

Regards https://github.com/profanity-im/profanity/issues/1578

src/config/preferences.c

@@ -2251,6 +2251,7 @@ _get_default_boolean(preference_t pref)
     case PREF_COLOR_NICK_OWN:
     case PREF_INTYPE:
     case PREF_INTYPE_CONSOLE:
+    case PREF_NOTIFY_MENTION_WHOLE_WORD:
         return TRUE;
     default:
         return FALSE;