No syntax highlighting or autocomplete in action

**Describe the bug**
When defining an action `.github/actions/[action-name]/action.yml`, there is no syntax highlighting for using variables and there's also no "help" from autocomplete for inputs, etc. like there is in a normal workflow.

**To Reproduce**
Steps to reproduce the behavior:
1. Create an action
2. Write some inputs
3. Use said inputs in steps
4. See error

**Expected behavior**
Proper syntax highlighting and autocomplete.

**Screenshots**

**Extension Version**
`v0.27.0`

**Additional context**
Add any other context about the problem here.