SkinVector class has an Icon map which maps menu items to their corresponding menu items:
The betafeatures key comes from Extension:betafeatures and all the other icons come from core.
Instead of having the map here, I suggest we continue the approach in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/SandboxLink/+/698066 and declare these icons at time of creation rather than Vector.
This will help us in future when we want to use such icons in other skins e.g. MinervaNeue