Ability to define custom values for existing properties / syntax for the custom properties

Currently, when adding new properties via a properties field, they're added without any values.

At least, we should have a way to add an array of potential values, both for existing properties, and for our custom properties.

Ideally, it would be awesome if we could define a syntax for the property to use — I see that the mdn-data-importer already has some utilities that could be used to parse it — why not allow this as a part of the css-data?

Right now developers have to use custom extensions when they want to provide custom values to their projects, having an ability to do this via css-data seems like the most straightforward way if it was possible, with an ability to generate such css-data.json files from existing design-tokens definitions and so on.

If this is something the team would be interested in and could provide guidance in how to best implement this — I would be happy to help.

Start by reviewing the css-data.json format and the mdn-data-importer utilities mentioned in the issue. The work is complete when the format can represent potential values and custom property syntax for both existing and added properties, with a documented path for generating css-data.json from design-token definitions.