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

Start by reviewing the existing Workflow authoring feature and how action.yaml or action.yml files are detected. The work is done when custom, especially composite, actions receive equivalent schema validation and authoring help, including input syntax highlighting.

Escrita pelo modelo de indexação a partir do texto da issue.