Add comments to package.json

Progressively upgrading dependencies in the JS world is a little different compared to Ruby.

When updating a Ruby gem inside a gemfile, you might want to leave notes. For example:

gem 'some-gem', '~> 1.2.3' # TODO: Update once we're on Ruby 3.1

The comment is useful since it shares knowledge related to upgrading this dependency.

Javascript, or JSON, works a little differently. No comments are allowed inside the package.json file. Instead to achieve roughly the same result you can do this:

...
"//": [
    "TODO: Upgrade stylelint-config-sass-guidelines once we're on Node 11+",
    "TODO: Upgrade mini-css-extract-plugin once we're on 12.13+ and webpack 5+",
    "TODO: Upgrade style-loader once we're on 12.13+ and webpack 5+"
  ],
"dependencies": {
  ....
}

"//" will never be used by npm for any purpose, and is reserved for comments.