fix(typescript): provide .d.ts files at the root

[tay1orjones]

Currently in the build output there is not an index.d.ts that references all the component's .d.ts files. Without this intellisense, etc does not work.

I've modified the root index.js to be .ts, which should get rollup to add the root index.d.ts. Unfortunately this has highlighted another issue - anywhere we're using

export default from './Component';

needs to be refactored to

import Component from './Component';
export default Component;

Testing / Reviewing

• cd into packages/react
• run yarn build
• view es folder output - an index.d.ts should be present alongside the compiled index.js

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[tay1orjones]

Closing in favor of #12787