fix(typescript): rename index.js to index.ts to expose some components

[haocheng6]

Closes #14165

Changelog

Changed

• Rename several index.js files to index.ts to make sure export statements in node_modules/@carbon/react/lib/index.d.ts of a TS React project can be resolved by the TypeScript compiler. (Reference: How TypeScript resolves modules)

(Note: This PR renames index.js files that only reference TS files. There are still many components that cannot be imported in a TS project. Fixing the problem for all components in a single PR may lead to a huge PR which would be hard to test and review, so I only fix the easiest ones in this PR.)

Testing / Reviewing

• CI should pass.
• Existing stories should work the same as before.
• Manually verify the library built on the new code exposes components like DatePickerInput by following the steps below.
  • Build the project: yarn build
  • Create a new TypeScript React project: npx create-react-app test-carbon-import-ts --template typescript
  • In the newly created project, install @carbon/react: npm i @carbon/react
  • Copy the built lib to the new project (replace the path to the path on your machine): cp -r /root/my-fork/carbon/packages/react/lib /root/tmp/test-carbon-import-ts/node_modules/@carbon/react/
  • Add the following code snippet to src/App.tsx in the new project and verify those components are imported correctly.

import {
  ComboBox,
  DatePickerInput,
  ExpandableSearch,
  FileUploader,
  Filename,
  FileUploaderSkeleton,
  FileUploaderButton,
  FileUploaderDropContainer,
  FileUploaderItem,
  FormGroup,
  Loading,
  NumberInputSkeleton,
  NumberInput,
  OverflowMenuItem,
  RadioButton,
  RadioButtonGroup,
  SelectSkeleton,
  Select,
  SelectItem,
  SelectItemGroup,
  TimePicker,
  TimePickerSelect,
} from "@carbon/react";

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[tay1orjones]

Thanks for this!