Syntax highlighting, typechecking, and building for the MMT language.
Create a directory for your MMT archives (e.g., called archives).
Clone MMT archives into that directory (e.g., by git clone https://gl.mathhub.info/MMT/urtheories.git urtheories).
Open your archives directory in Visual Studio Code
On Windows, you can do this by navigating to your archives directory using WIndows Explorer, right-clicking in an empty space there, and selecting Open with Code.
Alternatively and for other operating systems, open Visual Studio Code and select File -> Open Folder....
In the Visual Studio Code window you can now open arbitrary *.mmt files.
When you have an *.mmt file opened, you can press Ctrl+Shift+P to open the command pallette and seek typecheck and build.
In case of errors or bugs, a helpful command is Reload MMT.