fallbackError loses custom error types

[aecorredor]

Bug report

• I have checked other issues to make sure this is not a duplicate.

Describe the bug

When using fallbackError as described in the docs for checking custom error classes (extended from Error) something weird seems to happen where the custom class prototype is lost and instance of CustomError always returns false within fallbackError. I checked instance of CustomError within the actual resolver before throwing, and it does return true as expect. So, it's not a TS or ESBuild compilation issue.