[BUG] [typescript-fetch] TypeScript 3.6 breaks generated code

Created by: someone1

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
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Description

The 3.6 release of typescript has some breaking changes that break the generated code. Now the generated code ships with TypeScript 2.4, but some (or at least I) like to update this to the latest for any free optimizations that may come with it.

openapi-generator version

4.1.0

Suggest a fix

GlobalFetch was removed, so use it's namespaced version under WindowOrWorkerGlobalScope