Add workspace scans to Problems

[Perdolique]

Summary

Run Zero Reference: Scan Workspace to bring zero-reference findings from open and closed TypeScript/JavaScript files into Problems 😎👍 The scan works with CodeLens disabled and takes you straight to each declaration name 💨

• ✨ Scan all workspace folders with cancellable progress and a Clear Workspace Scan Results command.
• 🔎 Respect file exclusions and suppression comments while continuing to count references from excluded files.
• ♻️ Share pending analysis with CodeLens, keep cancellation independent, and preserve the global limit of four reference lookups.
• 📌 Publish results as one snapshot, keep the previous snapshot on cancellation or discovery failure, and mark surviving entries stale after edits.
• 🧾 Report incomplete checks and file counts, with original provider errors available through Show Output.
• ✅ Add Extension Host coverage for closed files, unsaved edits, multiple roots, missing workspaces, exclusions, cancellation, and snapshot updates.
• 📚 Document the commands and snapshot behavior, including the limits of the available project reference graph.

Related issues

Closes #3

Related to #2: includes the diagnostic surface needed for workspace scans.