Harness scaffold and Node.js implementor

[kraenhansen]

This PR add scaffolding for a test harness.

It is a stacked PR and I'll keep this drafted until #10 merge and I rebase this branch on the new main.

Merging this PR will:

• Add a new implementors/node directory with a Node.js specific harness, which will run each test file in the tests directory using the built-in test runner provided by Node.js, based on the structure of the file-system. Each test file is executed in a separate child process to provide isolation (of global state and potential process crashes).
• Add the first test: Exercising a simple assert function injected into the test's global object by the harness, I intent to use this pattern to provide functionality needed by a test, which might need calls to runtime-specific APIs for implementation.
• Add a GitHub Actions workflow, highly inspired by the ci.yml workflow from the nodejs/node-addon-api repo.