Add baseline border styles for cover block

These new styles have zero specificity and are easily overridden by theme or global styles. They however ensure there is some value set such that a user is more likely to be given immediate visual feedback when editing individual border properties.

packages/block-library/src/cover/style.scss

@@ -253,3 +253,10 @@ section.wp-block-cover-image > h2,
 	padding: 0.44em;
 	text-align: center;
 }
+
+// The following sets baseline border styles with zero specificity such that
+// when a user begins to alter cover block borders via the block support UI they
+// see immediately visual changes.
+html :where(.wp-block-cover) {
+	border: 0 solid currentColor;
+}