feat(gatsby-image): export TS interfaces of gatsby-image object types

[kripod]

Description

When passing optimized images through type-safe components, the need for using gatsby-image object types may emerge.

E.g.:

import Img, { FixedObject } from 'gatsby-image';

type UserProps = {
  avatar: FixedObject;
  // ...
};

export default function User({ avatar }: UserProps) {
  return (
    <div>
      <Img fixed={avatar} />
      {/* ... */}
    </div>
  );
}

[pieh]

This seems reasonable, thanks @kripod!

[joealden]

@kripod could you please explain to me why these need to be exported? I have been using these interfaces for a long time now without problems (when they weren't marked as exported). I can't find anything in the TypeScript docs on declaration files about this, so I wondered if you know why this is allowed?

To get a better understanding of what I mean, here is a screenshot of the named imports available to me with gatsby-image 2.1.1 (the version that contains this PR's commits):

As you can see, I can import and use the GatsbyImageProps interface in my own code, and as the file currently exists, this interface isn't marked as exported. This is what it was like for me previously in gatsby-image in relation to the FixedObject and FluidObject interfaces.

The only reason I can think this would have ever worked is because maybe TypeScript treats index.d.ts files specially? Maybe they export everything in them by default or something? I am just guessing though, as like I said, I can't find any documentation that suggests this is the case.