Support Custom GitHub Action authoring

**Is your feature request related to a problem? Please describe.**
As a developer of some GitHub Actions, I would like to have the same level of help on the `action.yaml` (or `.yml` file), especially for composite actions.

**Describe the solution you'd like**
Detect the action file and setup schema validation and the same behavior of the `Workflow authoring` feature adapt to it.

**Additional context**

Add syntax highlighting on input for example, and so on

首先檢視現有的 Workflow 撰寫功能，以及如何偵測 action.yaml 或 action.yml 檔案。當自訂 action，尤其是 composite action，獲得等效的 schema 驗證與撰寫輔助功能（包括輸入語法醒目提示）時，這項工作就完成了。

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