chore: remove code duplication in custom errors

[tinovyatkin]

In the current implementation AbortError and FetchError pretty much repeating the same code (plus some magic strings). This PR moves common functionality into a base class and removes code duplication.

It also removes isAbortError and uses instanceof FetchBaseError as those errors are our internal classes and always instantiating from it.

[Richienb]

We should keep the separate errors in their own files but importing from the base class instead.

Done

[tinovyatkin]

@LinusU

Although, have you checked that the stack traces actually looks identical after making this change? I might be wrong here, but I think that when you do Error.captureStackTrace in the parent class, referencing this.constructor might be referencing the parent constructor, which will leave the leaf class in the stack trace?

Not, it will be referencing the actual class constructor. This is how super works.

[xxczaki]

LGTM 👍

[JefferyHus]

LGTM