chore(winget): add workflow to auto-submit new versions to winget-pkgs

Triggers after the Release workflow completes, fetches the latest
release tag, and runs wingetcreate update --submit to push the updated
manifest to dvalfrid/winget-pkgs and open a PR against
microsoft/winget-pkgs automatically.

Requires WINGET_TOKEN secret (classic PAT, public_repo scope).