Added Media.AddChatLine to the new Lua API

Unmerged part of https://github.com/OpenRA/OpenRA/pull/5469.
It looks like we can't specify System.Color values as a default parameter value. Oh well.

In the short term we should just remove that parameter, as we don't bridge Color into lua yet.  In the longer term we could expose our HSLColor to lua, and then give it a default value of white so that we can specify it cleanly here.