Allow transforming from Group to Cover

[mtias]

This is a transformation we really need to offer. Right now if you want to transition a set of blocks into a cover you need to jump through some hoops.

One consideration to make, particularly for group variations (stack / row): should the transform modify the container block to cover or should it preserve the group and add a cover as an extra wrapper?

cc @apeatling @andrewserong

[Mamaduka]

This transformation is already supported - #30890, but there's a very specific isMatch check, so the option isn't always visible.

gutenberg/packages/block-library/src/cover/transforms.js

Lines 63 to 77 in 7f2978a

	isMatch: ( { backgroundColor, gradient, style } ) => {
	/*
	* Make this transformation available only if the Group has background
	* or gradient set, because otherwise `Cover` block displays a Placeholder.
	*
	* This helps avoid arbitrary decisions about the Cover block's background
	* and user confusion about the existence of previous content.
	*/
	return (
	backgroundColor ||
	style?.color?.background ||
	style?.color?.gradient ||
	gradient
	);
	},

Edit: I think there's no need to require background color after #31402.

[mtias]

Right! Seems it should be an irrelevant condition now.

[andrewserong]

Nice idea! I think I've got something working okay, I'll open up a PR shortly.

[andrewserong]

I've opened up a PR in #40212 — there was a bit of a decision involved in figuring out a default dimRatio that works well enough so that it's clear what's going on once a user starts interacting with adding images or background colors to the Cover block. Happy for any feedback over on that PR!