Add core-js polyfill for Promise.finally()

What does this MR do?

Adds a core-js polyfill for the Promise.finally() method.

Promise.finally() is a part of the latest ECMAScript draft specification. However, it isn't supported in IE 11 or Edge <= 17.

What are the relevant issue MR numbers?

https://gitlab.com/gitlab-org/gitlab-eslint-config/merge_requests/12#note_148522887

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer