Block style: ensure default for theme is properly reflected in the UI

[annezazu]

What problem does this address?

In chatting with @anarieldesign, she mentioned that, when creating additional block styles for buttons, no matter what style of the button you use as the default, it will always mark "Fill" as a default:

In the above example, the 3d button dark is actually the default but the UI doesn't reflect that.

What is your proposed solution?

Ideally, a block themer could determine which is the default used and that can be somehow marked in a way that the UI can respond appropriately. This is tangentially related to this issue around how best to show default values with design tools #43082

cc @WordPress/block-themers in case others have run into this or have any thoughts here!

[scruffian]

Similar to #42297