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).

首先跟踪用于发现全局和外部样式表的 ng-update 逻辑，然后检查 angular.json 中的 workspace 配置及其 styles 条目。完成的标准是，样式表迁移仅限于为项目声明的 styles，而不是 workspace 中的每个 .css 或 .scss 文件；该 issue 未指定测试文件或特定的源代码入口点。

由索引模型根据 Issue 内容生成。