GitHub Workflow file does not support YAML formatter.

**Describe the bug**
GitHub Workflow file does not support YAML formatter.

**To Reproduce**
Steps to reproduce the behavior:
1. Open a Workflow file.
2. Right click for the context menu
3. No "Format Document" available

**Expected behavior**
The plugin let VS Code know that Workflow file is in fact yaml and installed formatter can be used.

**Screenshots**

**Extension Version**
`0.27.2`

**Additional context**

VS Code:

Version: 1.102.0
Commit: cb0c47c0cfaad0757385834bd89d410c78a856c0
Date: 2025-07-09T22:10:34.600Z (6 days ago)
Electron: 35.6.0
ElectronBuildId: 11847422
Chromium: 134.0.6998.205
Node.js: 22.15.1
V8: 13.4.114.21-electron.0
OS: Darwin arm64 24.5.0

Reproduce the issue in VS Code 1.102.0 with extension version 0.27.2 by opening a GitHub Workflow file and checking the context menu. Review how the extension identifies Workflow files and verify that an installed YAML formatter becomes available through Format Document; done means the command appears for these files.

索引モデルが issue の本文から書いたものです。