ng-update: consult workspace configuration for determining global stylesheets

[devversion]

This is a port of COMP-280 and for just for tracking at this point!

Currently, ng-update always finds global/external stylesheets that need to be migrated by simply
querying for .css or .scss files in the workspace tree.

This is not ideal since it can mean that non-project files are accidentally migrated. Ideally, we'd just
consult the angular.json file to look for styles. This makes the logic more future-proof (and avoids issues as in: #18434).