Add a GitHub Action to run npm test

[cclauss]

Test results: https://github.com/cclauss/gpu.js/actions

• https://docs.github.com/en/actions/tutorials/build-and-test-code/nodejs
• https://github.com/actions/setup-node

[fuzzie360]

Thanks for kicking this off! The repo now has a CI workflow on develop (commit 2127766) that additionally builds via gulp and compares test results against a known platform-failure baseline — a plain npm test gate would be permanently red due to ~87 known Mesa/llvmpipe float-precision failures. Closing in favor of that workflow.