* Update nvmrc to Node.js 16
* Update package.json minimum Node engine to 16
* Update README requirements to Node.js 16
* Update devcontainer Node.js version to 16
* Update devcontainer Dockerfile Node.js choices to LTS versions that are still in support/maintenance
* Pin CircleCI Node image to 16
* Fix YAML type issue
* Update CircleCI Node.js to 16.18 to match #22019
* Improve devcontainer for running tests
- Pull devcontainer post-create out into its own script
- Add asset precompilation
- Add test-mode asset precompilation (needed to run tests without error)
* Document Gemfile.lock re-checkout in devcontainer
