Syntax highlighting and autocomplete for composite actions

[dukeofgaming]

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.