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.

首先检查 Angular CLI 如何读取当前工作区的 angular.json，以及现有命令如何注册。比较提议的 `ng projects` 和 `ng ls` 入口点，然后验证所选命令会列出工作区中的每个可用项目，并说明每个项目可以如何使用。

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