typescript-angular: optimize tree-shaking by disabling providers array when for providedIn: root is set

Created by: zbarbuto

Relates to https://github.com/OpenAPITools/openapi-generator/pull/120

This PR removes services from the providers array in @NgModules when providedInRoot mode is on.

Listing the providers in the providers array is not required if using providedIn: 'root' syntax. By excluding the providers from this array, the ones that are not used can be tree shaken out and excluded from the final build.

More info here:

https://angular.io/guide/providers#providedin-and-ngmodules

Without this change, all services are bundled in a final production bundle, regardless of if they are injected or not.

PR checklist

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cc @topce @akehir