Enhance block spacing gap config to also target the first block in a container

[jasmussen]

#33812 added a blockGap property to theme.json which enabled spacing blocks inside a container using a simpler set of rules:

[.container] > * + * {
	margin-top: var( --wp--style--block-gap );
	margin-bottom: 0;
}

In effect this simplifies margins to be between blocks rather than both before and after, simplifying first-child/last-child rules and margin collapsing.

However at the moment, the rule only targets n+1 blocks:

A fix would be to also output the following rule:

[.container] > * {
	margin-top: 0;
	margin-bottom: 0;
};

That would ensure there are no default margins interfering with the first block in a stack.

[jasmussen]

CC: @youknowriad, just as a followup to #33812 (comment).

[richtabor]

+1 — I noticed the same as I explored blockGap this morning.