Add types for TS

[rmariuzzo]

I would like to use this library in a TS project and have typings.

[rmariuzzo]

I added some definitions that looks promising:

The typing definition looks like this:

declare module 'styled-media-helper' {
  export default <M extends string>(media: Record<M, number>): {
    up: (media: M) => string
    down: (media: M) => string
    between: (mediaMin: M, mediaMax: M) => string
    only: (media: M) => string
  } => {}
}

[fleg]

PR are welcome