Backtick support for @styled-system/css

[JakeElder]

It would be useful to be have backtick style syntax support for @styled-system/css like emotion & styled-components, IE

const P = styled.p(
  css`
    font-size: 0;
    color: brand.bodyCopy;
    strong {
      font-weight: 500;
    }
  `
);

It would allow autocompletion via https://github.com/microsoft/typescript-styled-plugin

Ideally we could do something smart to enable auto completion for @styled-system magic too, IE typing

color: |<trigger autocomplete>

would present a list of css colors, in addition to objects in myTheme.colors