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.

Start by tracing the extension logic that only parses `.github/**/*.yml` during workflow authoring, and identify the entry points responsible for syntax highlighting and file discovery. Define how configurable paths and parsing options should be supplied, then verify that workflows in directories such as `composite/some-team-name/` receive the same authoring support.

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