Add support for `@value` variables

[r0skar]

In addition to exporting classNames, we can also export variables in CSS Modules. However, these are not included in the declaration file. Here is a minimal example of what I mean:

/* style.css */
@value bgColor: cadetblue;

/* index.ts */
import styles from './style.css'

// {bgColor: "cadetblue"}
console.log(styles)

Current behaviour:

/* style.css.d.ts */
// empty...

Expected behaviour:

/* style.css.d.ts */
export const bgColor: string