Allow RefObject<T> in reach__router innerRef. (#43534)

While https://reach.tech/router/api/Link describes innerRef as a func,
it is actually passed to React's HTMLAnchorElement's "ref" prop as such:

`<a ref={ref || innerRef} ...`

See: https://github.com/reach/router/blob/master/src/index.js#L392-L405

HTMLAnchorElement's "ref" is defined as Ref<HTMLAnchorElement>.

types/reach__router/index.d.ts

@@ -59,7 +59,7 @@ export interface LinkProps<TState> extends AnchorProps {
     getProps?: (props: LinkGetProps) => {};
     state?: TState;
     /** @deprecated If using React >= 16.4, use ref instead. */
-    innerRef?: React.RefCallback<HTMLAnchorElement>;
+    innerRef?: React.Ref<HTMLAnchorElement>;
 }
 
 export interface LinkGetProps {

types/reach__router/reach__router-tests.tsx

@@ -59,3 +59,6 @@ const handleRef = (el: HTMLAnchorElement) => {
 };
 
 render(<Link innerRef={handleRef} to="./foo"></Link>, document.getElementById('app-root'));
+
+const refObject: React.RefObject<HTMLAnchorElement> = { current: null };
+render(<Link innerRef={refObject} to="./foo"></Link>, document.getElementById('app-root'));