[styled-engine] Add missing type dependency on csstype

[Methuselah96]

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

The TypeScript declaration files for @mui/styled-engine import from csstype, so csstype should be added as a dependency of @mui/styled-engine.

TS2307: Cannot find module 'csstype' or its corresponding type declarations.
  > 1 | import * as CSS from 'csstype';
      |                      ^^^^^^^^^
    2 | import { StyledComponent, StyledOptions } from '@emotion/styled';
    3 | import { PropsOf } from '@emotion/react';
    4 |

[mui-bot]

No bundle size changes

Generated by 🚫 dangerJS against 278ed29

[michaldudak]

Thanks!