[system] Support style.transform return React.CSSProperties

[yoyooyooo]

• I have followed (at least) the PR section of the contributing guide.

transform should support map one to many css

import { style } from '@material-ui/system';

export default style({
  prop: 'flexCenter',
  cssProperty: false,
  transform: flexCenter => {
    return flexCenter
      ? {
          display: 'flex',
          justifyContent: 'center',
          alignItems: 'center',
        }
      : {};
  },
});

use :

<Box flexCenter>center</Box>

[mui-pr-bot]

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[oliviertassinari]

@yoyooyooo Nice one:

<Box flexCenter>center</Box>