core: @layer is not part of property hash

[layershifter]

const useClasses = makeStyles({
  root: {
    color: 'red',
  },
  rootWithLayer: {
    '@layer foo': {
      color: 'red',
    },
  },
});

Both color: red will have the same hash, this will cause collisions during merging.

https://stackblitz.com/edit/vitejs-vite-lvmmff