Support for core-js 3+

[VenkataJanapareddy]

Im using JSO in a Vue app built using vue-cli(v4.0.0) which has core-js (v3.3.2) as a dependency. But JSO is using core-js (v2.x.x). Hence , Im getting the following error.

These dependencies were not found:

* core-js/fn/array/includes in ./node_modules/jso/src/JSO.js
* core-js/fn/object/assign in ./node_modules/jso/src/JSO.js
* core-js/fn/promise in ./node_modules/jso/src/JSO.js

core-js has moved the above from core-js/fn to core-js/es. If I downgrade my core-js version to 2.x, the vue-cli wont work. Can we get support for core-js 3 since 2.x is deprecated?

[dosstx]

This pull request fixed the issue (for now):

#99