[fixed] Add file extention to entry point for TypeScript importing

[kenju]

Though this is not react-modal library's problem, here is a tweak fix necessary for TypeScript project.

Please see below post for more details:

http://stackoverflow.com/questions/41077055/typescript-2-1-feature-of-implicit-import-not-working

FYI

"Untyped imports" Section:
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html

// Succeeds if `node_modules/asdf/index.js` exists
import { x } from "asdf";

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[claydiffrient]

Released in 1.6.5

[kenju]

@claydiffrient
Thank you 👍