Add support for typed Custom CSS Properties

[p-ob]

Summary

Today, there's no way to provide type-hints to consumers about a CSS custom property (e.g. '<color>', '<length>', etc.) This requires component authors to have very explicit property names (--my-foo-color) or document the property adequately in the description. Unfortunately, neither is suitable for IDE intellisense support.

Chrome has shipped the CSS @property rule and enabled by default as of 85, and the CSS Properties and Values API as of 78.

Proposal

Add a "type" property to the Custom CSS Property schema. Manifest consumers can treat the omission of this value as '*', which I believe is the default behavior of Custom CSS Properties.

We can additionally strongly type the TypeScript declaration schema with all the legal string values

[justinfagnani]

This is a good idea. I think we should add a "syntax" field to more closely match the properties and values APIs. I wish we had used "initialValue" rather than "default" for that reason.