cross-ci/lib/var/IS_PR.d.ts

`cross-ci` standardizes environment variables for CI. For example, your can simply use `BUILD_BRANCH` variable in all CI runners instead of `CIRCLE_BRANCH` in CircleCI or `TRAVIS_PULL_REQUEST_BRANCH` in Travis.

import { ICrossCIVars } from '../createVars';
declare const IS_PR: ({ BUILD_PR_NUM }: ICrossCIVars) => boolean;
export = IS_PR;