Support Custom GitHub Action authoring

**Is your feature request related to a problem? Please describe.**
As a developer of some GitHub Actions, I would like to have the same level of help on the `action.yaml` (or `.yml` file), especially for composite actions.

**Describe the solution you'd like**
Detect the action file and setup schema validation and the same behavior of the `Workflow authoring` feature adapt to it.

**Additional context**

Add syntax highlighting on input for example, and so on