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.

Reproduce the issue in `.github/actions/[action-name]/action.yml` using the listed steps, then inspect how the extension provides syntax highlighting and autocomplete for normal workflows versus action definitions. Done means variables and action inputs receive equivalent highlighting and autocomplete support in action files; verify this with the reproduction steps and an appropriate extension test if available.

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