Support Babel 8

Babel 8 is now stable, but `babel-plugin-styled-components@2.3.0` currently declares:

```json
"peerDependencies": {
  "@babel/core": "^7.0.0"
}
```

This causes `npm install` to fail with `ERESOLVE` when using `@babel/core@8.x`.

Could Babel 8 support be tested and, if compatible, the peer dependency be updated to something like:

"@babel/core": "^7.0.0 || ^8.0.0"

Thanks!