Add devcontainer config file for use with Codebases

[DylanSp]

https://docs.github.com/en/codespaces/setting-up-your-project-for-codespaces/adding-a-dev-container-configuration/introduction-to-dev-containers#creating-a-custom-dev-container-configuration

• VS Code extensions:
  • ESLint - dbaeumer.vscode-eslint
  • Prettier - rvest.vs-code-prettier-eslint
• color theme: Dark+
• See if I can load bash dotfiles from https://github.com/DylanSp/dotfiles
  • Make sure Git commit editor on command line uses nano (should be handled by setting EDITOR env var)

[DylanSp]

Some of this may fall under personalizing Codespaces, rather than having configuration for this repo - see https://docs.github.com/en/codespaces/customizing-your-codespace/personalizing-github-codespaces-for-your-account. Quite possibly, the only config that should be in this repo's devcontainer.json should be the extensions (which I should recommend in this repo's .vscode directory - see #17).

[DylanSp]

Looking at https://docs.github.com/en/codespaces/setting-up-your-project-for-codespaces/adding-a-dev-container-configuration/setting-up-your-nodejs-project-for-codespaces#step-2-add-a-dev-container-configuration, I may be able to base my config on the existing Node templates. Main changes I'd want to make:

• Make sure git is installed (is hopefully included by default)
• Install extensions - see https://containers.dev/supporting#visual-studio-code
• Set postCreateCommand to npm ci, to install dependencies