Add constructible stylesheet as an output format

As a developer who often works with custom elements and shadow DOM, I would love to have access to open-props using [constructible stylesheets](https://web.dev/articles/constructable-stylesheets) as an output target. I'd be happy to contribute this feature if there's interest but limited bandwidth.

Start by reviewing the existing open-props output targets and the linked constructible stylesheets reference to determine the required format and integration points. Done means open-props can be consumed as a constructible stylesheet output target, with its behavior documented and verified against the existing outputs.

Written by the indexing model from the issue text.