This is a simple syntax highlighter for this language named SunlightLang. It is exactly what it sounds like, an extension that highlights syntex for SunlightLang. I have refrained from publishing it on the VSCode marketplace for this moment because I believe that this extension currently isn't production ready, as it may frequently be updated, to add/remove features just because the language it's highlighting is doing the same.
To install this you should click on the extensions tab on VSCode, and there would appear a menu where an option of installing from VSIX will be avalible, just click that and select this vsix file from this repository. That would simply install this extension into your VSCode installation.