Fix sibling inserter hit area (#9229)

This fixes #8881.

The sibling inserter hit area was not centered perfectly between two blocks. This PR fixes that.

Props to @chrisvanpatten for this fix.

packages/editor/src/components/block-list/style.scss

@@ -718,6 +718,7 @@
 .editor-block-list__insertion-point {
 	position: relative;
 	z-index: z-index(".editor-block-list__insertion-point");
+	margin-top: -$block-padding;
 }
 
 .editor-block-list__insertion-point-indicator {
@@ -729,6 +730,7 @@
 	background: theme(primary);
 }
 
+// This is the clickable plus.
 .editor-block-list__insertion-point-inserter {
 	// Don't show on mobile.
 	display: none;
@@ -740,13 +742,8 @@
 	bottom: auto;
 	left: 0;
 	right: 0;
-	// Matches the whole empty space between two blocks
-	height: $block-padding * 2;
 	justify-content: center;
 
-	// Position above block.
-	top: -$block-padding;
-
 	// Show a clickable plus.
 	.editor-block-list__insertion-point-button {
 		margin-top: -4px;
@@ -786,6 +783,7 @@
 	}
 }
 
+// This is the edge-to-edge hover area that contains the plus.
 .editor-block-list__block {
 	> .editor-block-list__insertion-point {
 		position: absolute;