Add type parameter for ref objects

All HTML elements are descendants of HTMLElement, but they also may have additional methods. For this to work right, we need the option to specify what exactly the element is expected to be.

types/airbnb-prop-types/index.d.ts

@@ -162,7 +162,7 @@ export function range<T extends number>(min?: number, max?: number): PropTypes.R
 
 export function range(min?: number, max?: number): PropTypes.Requireable<number>;
 
-export function ref(): PropTypes.Requireable<ReactLegacyRefLike<HTMLElement>>;
+export function ref<T = HTMLElement>(): PropTypes.Requireable<ReactLegacyRefLike<T>>;
 
 export function requiredBy<P>(
     requiredByPropName: string,