Fix corresponding type declarations

[hywax]

Need to add a "types" field to solve the TS problem

TS2307: Cannot find module conf or its corresponding type declarations.

It solves issue sindresorhus/electron-store#276

[sindresorhus]

This is not correct. If you have configured your project correctly, TypeScript will read the types location from the exports field. Adding "types" may seem like a workaround, but it could have unintended consequences. The current package.json is correct.