`Required property is missing: shell`

**Describe the bug**

The VS Code extension lacks critical linting support. Specifically, when creating a Composite Action, it fails to provide a warning that a `shell` property is required. I only became aware of this after pushing an update and encountering the following error:

`.github/actions/aws-config/action.yml (Line: 18, Col: 7): Required property is missing: shell`

**Expected behavior**
The VS Code extension should display a warning when authoring a Composite Action for each step that meets the following conditions:
- `run` property is used
- `shell` property is not specified

**Extension Version**
`v0.26.3`