Add Message::listParam()

Add Message::listParam()

This allows for passing a list of values that will be turned into a list
in the context of the language for which the Message is being processed.

For example, currently you'd have to do

 $msg = new Message( 'something', [ $language->commaList( $list ) ] );

which isn't going to give correct results if the message is later
changed to a different language with a different value for
'comma-separator'.

Now, you can do this instead

 $msg = new Message( 'something', [ Message::listParam( $list, 'comma' ) ] );

and it will be listified properly no matter what language is later used to
parse $msg.

Change-Id: I66868c61832260870449998fef14c842f17753ee