Revert "Do not overwrite existing vars on export"

This reverts commit 9039ac8c839c723847bbd984351a6f580cf57f8d.
Remove existing object check from global UMD

This is something that libraries can implement if they choose, but if we implement it’s not possible for them to make this choice and it prevents things like noConflict mode.
Update release notes
v2.0.0