fix(TS): Add toBeVisible declaration (#20)

Author: smacpherson64

extend-expect.d.ts

@@ -4,6 +4,7 @@ declare namespace jest {
     toHaveTextContent: (text: string) => R
     toHaveClass: (className: string) => R
     toBeInTheDOM: () => R
+    toBeVisible: () => R
     toHaveStyle: (css: string) => R
   }
 }