Add dependency security maintenance

[tnotheis]

Summary

• add the reusable dependency security maintenance workflow
• add an .nsprc config file for better-npm-audit exception management

Notes

This mirrors the dependency security maintenance setup added in js-soft/ts-serval commit 2b810501207b3e12578de2f9dc0bdd3888d06095. ts-utils already runs npx better-npm-audit audit without inline excludes, and its current audit is clean, so .nsprc is intentionally empty.

Validation

• bash .ci/runChecks.sh
• npm run build
• npm run test