Added Media.AddChatLine to the new Lua API

Unmerged part of https://github.com/OpenRA/OpenRA/pull/5469.
I think the best name for this would be `Media.DisplayMessage`.

This currently works by adding a chat line, but there is a long term plan to make system message (from the map, "base under attack", "selected all on screen", "New Objective: Capture the prison", etc) display with distinct formatting from chat.