Don't enforce "skipLibCheck: true"

[parzhitsky]

Follow-up after #69

Current export structure uses @ts-ignore to suppress conflicting export = and export type {}; in src/index.ts, however both are needed to properly support CommonJS's const xrange = require("xrange"); as well as TypeScript's import xrange from "xrange"; syntax.

Unfortunately though, the // @ts-ignore comments is stripped out on build (and it should be frankly), which produces error in node_modules folder of dependent projects. Since there's no way of fixing this error (this would require manually changing code in node_modules), the only (adequate) way to suppress the error is to set skipLibCheck: true compile option.

Find a fix (probably, wait for microsoft/TypeScript#38866 to be implemented) that would remove the requirement of skipLibCheck: true