Add generic typing ability

[flextremedev]

Provide an API for checking more specific components via Props and/or State.
For example:

let maybeFC: React.FC<{foo: string}> | string = ({foo}) => <div>{`${foo}, I'm a function component!`}</div>;
if (Math.random() > 0.5) {
  maybeFC = 'Not FC';
}
if (isFunctionComponent<{foo: string}>(maybeFC)) {
  const fcWithProps: React.FC<{foo: string}> = maybeFC;
  const FCWithProps = fcWithProps;
  const fcWithPropsElem = <FCWithProps foo='bar'}/>; // should not have a type error
} else {
  const notFC: string = maybeFC;
}