What?

Add a way for a block's variations to not render within the block card, while also maintaining transformability between each other. A follow-up to support #55520, where we don't need the transforms from H1 to H6 in the block inspector. This would also help with leaning in to variations more, especially around content -> content transformations.

This PR adds support to the "Date" block's "Modified Date" variation.

Note that hidden may not be the best scope value; open to suggestions.

Testing Instructions

1. Add the "Date" block to a page.
2. See the "Modified Date" block variation within the "Transform to" popover, but not in the block card (sidebar inspector).

Screenshots or screencast

Before	After