List available projects in a workspace - `ng projects`

# 🚀 Feature request

### Command (mark with an `x`)

- [X] projects

### Description

In large Angular monorepo workspaces, it can sometimes be non-obvious to discover all the available projects which can be built. We should consider a new command (`ng projects`, `ng ls`, etc.) which lists all the projects in the current workspace. This can help new developers in workspace figure out exactly what they want to work on and how to do it.

### Describe alternatives you've considered

Workaround is to introspect the `angular.json` file, which is not obvious or easy to do, especially for a new developer.

首先檢查 Angular CLI 如何讀取目前工作區的 angular.json，以及現有命令如何註冊。比較提議的 `ng projects` 和 `ng ls` 進入點，然後驗證所選命令會列出工作區中的每個可用專案，並說明每個專案可以如何使用。

由索引模型根據 Issue 內容生成。