Add dependency security maintenance

[tnotheis]

Readiness checklist

• I ensured that the PR title is good enough for the changelog.
• I labeled the PR.
• I added tests (if necessary).
• I self-reviewed the PR.

Summary

• add the reusable dependency security maintenance workflow
• add an .nsprc config file for better-npm-audit exception management
• migrate inline better-npm-audit exclusions into .nsprc where present

Template

Mirrors js-soft/ts-serval@3b61a40.