Adds TypeScript definition file

[thiagodp]

The only exception is delete(), because it is recognized as a reserved word.

[SamVerschueren]

There are already type definitions published to DefinitelyTyped, you can install them with npm install @types/dot-prop. More info on why most of the time, we don't add type definitions to our projects can be found here sindresorhus/ama#439.

[thiagodp]

@SamVerschueren Thanks for pointing this out.

[sindresorhus]

I now accept TypeScript definitions if anyone looking is interested in submitting it here.

See: sindresorhus/ama#439 (comment)

Please follow this guide: https://github.com/sindresorhus/typescript-definition-style-guide