chore(nodejs): Node 20, tooling, node:test, testing guide

[vkozio]

Summary

• Require Node 20+ for the Node.js addon; update cmake-js and node-addon-api.
• Migrate tests to Node.js built-in test runner (node:test).
• Add testing guide for authors in tools/nodejs_api/docs/nodejs_testing.md.

Motivation

Align with Discussion #135 (v0.15.x); drop legacy test runners; document how to write and run Node tests.

Changes

• tools/nodejs_api: package.json (engines, deps), CMake, build scripts, tests, docs/nodejs_testing.md.
• docs/testing.md: link to Node testing guide.
• No CI in this PR; CI can be proposed separately if needed.

Testing

• make nodejs && cd tools/nodejs_api && npm test