Add type definitions for netlify-identity-widget

[nkprince007]

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

If adding a new definition:

• The package does not already provide its own types, or cannot have its .d.ts files generated via --declaration
• If this is for an NPM package, match the name. If not, do not conflict with the name of an NPM package.
• Create it with dts-gen --dt, not by basing it on an existing project.
• tslint.json should be present, and tsconfig.json should have noImplicitAny, noImplicitThis, strictNullChecks, and strictFunctionTypes set to true.

[nkprince007]

As suggested by the package maintainer (@sw-yx) of netlify-identity-widget, the type definitions are expected to be maintained here.

[typescript-bot]

@nkprince007 Thank you for submitting this PR!

Because this is a new definition, a DefinitelyTyped maintainer will be reviewing this PR in the next few days once the Travis CI build passes.

In the meantime, if the build fails or a merge conflict occurs, I'll let you know. Have a nice day!

[typescript-bot]

@nkprince007 One or more reviewers has requested changes. Please address their comments. I'll be back once they sign off or you've pushed new commits or comments. Thank you!

[nkprince007]

Thanks for the early review @uniqueiniquity. The response has been extremely timely. 👍

[typescript-bot]

🔔 @uniqueiniquity - Thanks for your review of this PR! Can you please look at the new code and update your review status if appropriate?

[uniqueiniquity]

Congratulations on your first DefinitelyTyped contribution!
Thank you for being a part of the community!