ng-update: consult workspace configuration for determining global stylesheets

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).

Start by tracing the ng-update logic that discovers global and external stylesheets, then inspect the angular.json workspace configuration and its styles entries. Done means stylesheet migration is limited to styles declared for projects rather than every .css or .scss file in the workspace; the issue does not name a test file or specific source entry point.

Written by the indexing model from the issue text.