Buttons: Support gap via theme.json

[jasmussen]

What problem does this address?

#32366 adds a theme.json mechanism to provide a a blockGap value to space out items in a container. Spacing using gap has the benefit of letting you not worry about the first or last elements in a container, or complex nth-child margins in containers that wrap.

What is your proposed solution?

As a next step, it would be nice to connect the block with theme.json, so that the block becomes a flex container with a custom gap value, such as this:

"core/buttons": {
	"spacing": {
		"blockGap": "2em"
	}
}

#33991 also provided a UI for the gap control, which would benefit here as well.

[andrewserong]

There's a PR in progress that will address this — #34546 (thanks for picking that up, @stacimc!)

[bradhogan]

This appears to be added now, but it's not working. Whatever blockGap I define in my theme.json is ignored.

[andrewserong]

@bradley2083 there's a bit more context in #39789, which is the current issue for tracking adding back in support for blockGap at the block level within theme.json. (There is a WIP PR to add the support in #39870)