[styled-components](https://github.com/styled-components/styled-components) is a widely used CSS-in-JS library. It lets users write their CSS in JavaScript as CSS strings. For example:

```
const Button = styled.button`
  color: blue;
  background: red;
`
```

This PR adds support for highlighting the styled-components template strings as CSS.

We have been running this exact code [on the styled-components website for almost two years](https://github.com/styled-components/styled-components-website/blob/bbfcb0d21aaec9e8202609a67e7bba5278042056/utils/prismTemplateString.js) and it has worked perfectly.

I have added tests for the feature and am willing to maintain this going forward.

 Added `createTemplate`