Support Unicode encodings

It would be convenient if Millfork allowed string literals for UTF-8 and UTF-16. This might seem like an odd feature to have, but I've been working on supporting rendering Unicode text on the Commander X16, especially CJK text. UTF-8 would also make sense for platforms such as the Altair 8800 where encoding is mostly a matter of agreement between the software running on the computer and the terminal being used to access it.