[typescript-angular] Change RxJS imports, if using Angular >= 6

[dennisameling]

Currently, there is no support for RxJS 6 yet in the 3.0.0 branch of Swagger codegen (the master branch already has it through this commit. That commit should be copied to this repo, so that the behavior is consistent for OAS 3.0.0.

[dennisameling]

Duplicate PR of #236, didn't see it before as there's no issue linked to it in this repo

[karismann]

hello @dennisameling I suggest you to take a look at the forked project https://github.com/OpenAPITools/openapi-generator where it is already supported. It also generate typescript angular API client in v7.