Co-authored-by: Effy Elden <effy@effy.space>

* 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

* Format JSON and YAML using Prettier

* Add prettier to devDep

* Add DevContainer config

* Add `--path vendor/bundle` to `postCreateCommand`