Pin babel-preset-react-app to to core-js minor '3.5' version and add a peer dependency

Created by: Mrman

• Specified the minor version as recommended by core-js docs
• Removed comment about core-js@3 upgrade
• Added peer dependency on core-js from babel-preset-react-app.

Did some due dilligence and see react-error-overlay and react-scripts both directly depend on babel-preset-react-app but do not have a direct dependency on core-js. They do both have direct dependencies on react-app-polyfill that in turn depends on core-js.

From my testing:

• npm doesn't complain about the peer depenency being unmet, guessing because it sees core-js in the dependency tree from react-app-polyfill
• yarn does warn about the peer dependency being unmet, this seems more correct than npm's behaviour.

Forgive my ignorance but I don't know if babel-preset-react-app is expected to be usable without core-js? If not does that suggest react-scripts and react-error-overlay should have a direct dependency on core-js too?

closes #8875 (closed) closes #8779