upgrade core-js to 3.x

[lygstate]

Prerequisites

• I have read the documentation;
• In the case of a bug report, I understand that providing a SSCCE example is tremendously useful to the maintainers.
• Ideally, I'm providing a sample JSFiddle or a shared playground link demonstrating the issue.

Description

[Description of the bug or feature]

Steps to Reproduce

1. [First Step]
2. [Second Step]
3. [and so on...]

Expected behavior

[What you expected to happen]

Actual behavior

[What actually happened]

Version

You can usually get this information in your package.json or in the file URL if you're using the unpkg one.

[epicfaace]

Sounds good. Would you like to make a PR doing this @lygstate ?