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.

Start by locating the existing workflow syntax-highlighting and autocomplete entry points, then compare their handling with .github/actions/**/action.yml and root action.yml. Done means composite action files receive syntax highlighting and autocomplete comparable to workflows; verify both example locations.

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