Define babel corejs version for useBuiltIns

[dschissler]

For example:

WARNING: We noticed you're using the `useBuiltIns` option without declaring a core-js version. Currently, we assume version 2.x when no version is passed. Since this default version will likely change in future versions of Babel, we recommend explicitly setting the core-js version you are using via the `corejs` option.

You should also be sure that the version you pass to the `corejs` option matches the version specified in your `package.json`'s `dependencies` section. If it doesn't, you need to run one of the following commands:

  npm install --save core-js@2    npm install --save core-js@3
  yarn add core-js@2              yarn add core-js@3

[dschissler]

PDF.js isn't compatible with core-js 3 since they are importing the functions directly. Due to this and probably other libraries hard dependencies its looking like a hard break will be happening in the future with some degree of holding back and measured compromises.