Syntax highlighting for repos that put composite workflows in directories other than `.github`

**Is your feature request related to a problem? Please describe.**
It's often nice to put shared workflows in directories that make sense for consumers, like `my-private-org/composite/some-team-name/their-reusable-workflow.yml`. The extension works fine for _resolving_ these workflows when they're used, but it doesn't syntax highlight or do anything when _authoring_ these workflows. It seems to pay attention to `.github/**/*.yml` exclusively.

**Describe the solution you'd like**
Configurable paths and options for which files should be parsed.

**Additional context**
Add any other context or screenshots about the feature request here.