Restructure example-node into a single calculator app (fixes all Dependabot alerts)

[thomasrockhu-codecov]

Summary

Modernizes example-node to mirror the structure of example-python: a single calculator app with runnable tests, wired up to Codecov via GitHub Actions.

This also resolves all 31 open Dependabot alerts — not by bumping transitive pins, but by removing their source. Every alert came from the deprecated codecov npm uploader (→ teeny-request → uuid/js-yaml, → http-proxy-agent → @tootallnate/once) and the abandoned istanbul/old mocha trees. The new stack has 0 vulnerabilities.

Changes

• App: added app/calculator.js (add/subtract/multiply/divide) and app/calculator.test.js, mirroring example-python.
• Test + coverage: uses the built-in Node.js test runner (node --test, zero deps) plus c8 for lcov coverage and a JUnit report. npm test runs the suite, writes coverage/lcov.info, and junit.xml.
• CI: replaced .travis.yml with .github/workflows/ci.yml (checkout → setup-node → install → test → upload coverage/test-results to Codecov, including OIDC).
• Dependabot: added .github/dependabot.yml for the npm and github-actions ecosystems.
• Config/docs: added codecov.yml, rewrote README.md, updated .gitignore.
• Removed: the entire istanbul-mocha/ tree, the deprecated codecov uploader, .travis.yml, and the stray root package-lock.json.

Test plan

• npm install installs cleanly (c8 only) with 0 vulnerabilities.
• npm test passes (4 tests) and produces coverage/lcov.info + junit.xml.
• GitHub Actions runs green and uploads coverage to Codecov.
• Dependabot re-scan closes all 31 alerts.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@30d57f6). Learn more about missing BASE report.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
app/calculator.js	90.90%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage        ?   90.90%           
=======================================
  Files           ?        1           
  Lines           ?       22           
  Branches        ?        0           
=======================================
  Hits            ?       20           
  Misses          ?        2           
  Partials        ?        0           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.