Support Custom GitHub Action authoring

[rlespinasse]

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

[gs-emp]

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-using-an-action-in-the-same-repository-as-the-workflow

Seems like it should at least support using actions as defined in the github docs