I'm not sure what makes it look “like most ST extension authors are now moving to vscode”. New packages are submitted at a rate of several per week.
Here is the syntax definition documentation.
It looks like the existing Nunjucks package was only maintained for a short time and has been abandoned for years. It also looks like that syntax definition is based on a very old version of the core HTML definition. If I were you, I'd start from scratch rather than trying to extend the old syntax.
If you do create a better Nunjucks package, and the author of the original agrees, the “Nunjucks” name in Package Control can be moved to point to the newer package instead.