Social Icons: 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.

#34493 is already refactoring the social links block to use the new blockGap value.

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/social-links": {
	"spacing": {
		"blockGap": "2em"
	}
}

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