Inside Visual Studio Code, navigate to the Extensions view (Ctrl+Shift+X or Cmd+Shift+X).
Click the ... (Views and More Actions...) menu in the top-right corner of the panel.
Choose Install from VSIX...
Select the .vsix file you downloaded.
About Solarus
Solarus is a lightweight, multi-platform, free and open-source 2D game engine written in C++. It runs games scripted in Lua and is primarily designed for creating action-RPGs, also called Zelda-likes, but can do much more.