release: append NBB banner to all code files in the release package

[christophercr]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/ngx-form-errors/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

The code files contained in the final bundle generated after every release don't contain the NBB banner.

Expected behavior

The NBB banner should be present in all code files contained in the release package.

Minimal reproduction of the problem with instructions

Generate a release via the npm run release command

What is the motivation / use case for changing the behavior?

NBB branding

Environment

NgxFormErrors version: 1.0.0-alpha

[christophercr]

Apparently this was supported in since versions 2.0.0 and 2.1.0 of ng-packagr but unfortunately this feature was removed in 3.0.0 due to compliance with the new Angular Package Format (APF) v6.0.

So it seems we need to find a way to do this ourselves...