Autocomplete or hinting for reusable workflows

**Is your feature request related to a problem? Please describe.**
`uses` block doesn't have a functioning autocomplete which is frustrating when you have github recognized reusable workflows.

**Describe the solution you'd like**
When cursor is within a `uses` block, and you request an autocomplete the extension should populate with reusable workflows present in .github/workflows directory.

** Visualization of unhelpfulness **

Start by locating the completion handling for a `uses` block in the VS Code GitHub Actions extension, then inspect how reusable workflows are discovered in `.github/workflows`. The work is done when autocomplete lists the reusable workflows in that directory while editing a `uses` block.

Written by the indexing model from the issue text.