Add centering style also to the theme style output

Fixes comment in #5209 (comment).

Previous implementation assumed the theme would handle centering, just like it's traditionally handled alignleft and alignright classes, but since the aligncenter is now applied to the `figure` element, any theme that uses code like `img.aligncenter` won't work. Hence this PR.

blocks/library/image/style.scss

@@ -1,6 +1,12 @@
-.wp-block-image figcaption {
-	margin-top: 0.5em;
-	color: $dark-gray-300;
-	text-align: center;
-	font-size: $default-font-size;
+.wp-block-image {
+	figcaption {
+		margin-top: 0.5em;
+		color: $dark-gray-300;
+		text-align: center;
+		font-size: $default-font-size;
+	}
+
+	&.aligncenter {
+		text-align: center;
+	}
 }