Add Chrome, Edge and Opera partial support to css-subgrid.json

[Baptistou]

See issue here and here.
CSS subgrid does not work on nested fieldset elements on Chromium.
Issue reproduced today on Chrome, Edge and Opera latest version.

[Fyrd]

Hmm, this seems like it's too specific to warrant partial support, maybe should just be added to the bugs list?

[Baptistou]

Hmm, this seems like it's too specific to warrant partial support, maybe should just be added to the bugs list?

If I compare with :disabled attribute for example, I see that IE has partial support on fieldset --> fieldset-disabled.json.
So we could either modify css-subgrid.json to add partial support on Chromium-based browsers or create a new file fieldset-subgrid.json as above. What do you think?