update `typescript-fetch` for modern TS compat

Description

#6894 undid the inclusion of isomorphic-fetch and replaced it with portable-fetch (a fork of isomorphic-fetch, but without TypeScript types and with an incremental 3 line commit 3 years ago to support react native via fetch-npm-react-native.js.

Problem is, these declaration files simply don't work on modern typescript.

A better solution is to move to a maintained form of isomorphic-fetch which today is cross-fetch – which also (fortunately) includes types (and gets rid of the declaration file).

Swagger-codegen version

3.0.20

Command line used for generation

java -jar swagger-codegen-cli generate -l typescript-fetch -i foo.json

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/pull/6894

Suggest a fix/enhancement

1. replace with cross-fetch
2. remove declaration file

also ought to mention the last person who changed this... @jeff-99

Start with the typescript-fetch generator entry point and the declaration file used by generated clients, then run the documented Java command with a sample OpenAPI input to reproduce the modern TypeScript compatibility problem. Done means generated clients use cross-fetch instead of portable-fetch and no longer include the incompatible declaration file.