Add syntax Highlighting for BYOND DreamMaker

Problem to solve

The language DM, for the BYOND game engine, has no syntax highlighting here as it does on GitHub.

Intended users

Users who write code in the BYOND DreamMaker language.

Further details

None

Proposal

Add the proper syntax highlighting for the language.

Permissions and Security

It is UI based.

Documentation

I'm not sure how to document a change like this.

Testing

It doesn't really pose any risks, even if the highlighting turns out bad; it's better than nothing.

What does success look like, and how can we measure that?

Success is when you can look at a DreamMaker .dm file in a GitLab repository, and the syntax is highlighted.

Links / references