select and drop down don't tree-shake igxForOf

[damyanpetev]

Description

The Select and Dropdown components use declarative items that are normally iterated. The Dropdown optionally support those items to be iterated using the igxFor directive to create a virtualized dropdown items list - for that reason it references directly the IgxForOfDirective to react if it's used and that prevents it from being tree-shaken out even when it's not needed. The Select component by extending the Dropdown inherits the same issue.

Steps to reproduce

1. Create an app with just a dropdown or select component
   For example in the bundle-test project in the repo:
   
2. Build the app
3. Run source-map-explorer on the produced main chunk and inspect the result

Result

Expected result

The igxFor directive and supporting classes should not be included () in the resulting build bundle.
The components should reference the directive using a lightweight injection token (https://angular.io/guide/lightweight-injection-tokens) to allow it to be tree-shaken.