core: Support object literal with multiline keys

[gaoqiangz]

const useStyles = makeStyles({
    root: {
        [`
            & .item:hover,
            & .item-selected
        `]: {
            backgroundColor: '#fff',
        }
    }
});

[layershifter]

FYI, following works:

makeStyles({
  root: {
    [`& .item:hover, & .item-selected`]: {
       backgroundColor: '#fff',
    }
  }
});

[Shubhdeep12]

Hey @layershifter how about parsing the slots styles first and as you suggested above, fixing the property by something like this - property.replace(/(\r\n|\n|\r)/gm, " ").replace(/\s+/g, " ").trim()

this will remove the multiline and extra spaces