Autocomplete or hinting for reusable workflows

**Is your feature request related to a problem? Please describe.**
`uses` block doesn't have a functioning autocomplete which is frustrating when you have github recognized reusable workflows.

**Describe the solution you'd like**
When cursor is within a `uses` block, and you request an autocomplete the extension should populate with reusable workflows present in .github/workflows directory.

** Visualization of unhelpfulness **

先找出 VS Code GitHub Actions 擴充功能中對 `uses` 區塊的補全處理，接著檢查如何探索 `.github/workflows` 中的可重複使用工作流程。在編輯 `uses` 區塊時，自動完成會列出該目錄中的可重複使用工作流程，即表示工作完成。

由索引模型根據 Issue 內容生成。