Syntax highlighting and autocomplete for composite actions

**Is your feature request related to a problem? Please describe.**
Syntax highlighting works great when working on workflows, but dowsn't work at all when working on composite actions.

**Describe the solution you'd like**
Add syntax higlighting when working on a composite action (e.g. under .github/actions/**/action.yml, or root action.yml)

**Additional context**
This almost discourages working on composite actions over reusable workflows, even though reusable workflows can be more painful.