Parallel steps in GitHub Actions [GA]

### Value Prop

GitHub Actions will address the long-standing community ask to support running multiple steps in parallel in GitHub Actions

### Expected Outcome

There will be two primary feature components:
1. The ability to designate some steps as `background` steps, which will allow them to run without blocking the start of subsequent steps. We will also add additional `wait` steps where users can synchronize sequential execution and `cancel` steps that allow the graceful termination of long running `background` steps
2. A `parallel` block that will enable customers to designate a set of steps that will run in parallel. This block will be syntactic sugar that uses `background` steps and a `wait` command under the hood.

首先阅读 issue 的“Expected Outcome”，尤其是其中提出的后台、等待、取消和并行组件。没有指定 repository 文件、测试或 entry point；要完成此工作，需要实现并验证此处描述的 GitHub Actions 功能设计。

由索引模型根据 Issue 内容生成。