Template Part block - add border states in editor. (#24498)

* add border for selected, child selected, and hover states on template part block

packages/block-library/src/template-part/editor.scss

@@ -84,3 +84,37 @@
 		box-shadow: 0 0 0 $border-width var(--wp-admin-theme-color);
 	}
 }
+
+.block-editor-block-list__block[data-type="core/template-part"] {
+	&.is-selected,
+	&.has-child-selected {
+		&::after {
+			top: $border-width;
+			bottom: $border-width;
+			left: $border-width;
+			right: $border-width;
+			border-radius: $radius-block-ui - $border-width; // Border is outset, so so subtract the width to achieve correct radius.
+		}
+	}
+
+	&.is-selected {
+		&::after {
+			// 2px outside.
+			box-shadow: 0 0 0 $border-width-focus var(--wp-admin-theme-color);
+			// Show a light color for dark themes.
+			.is-dark-theme & {
+				box-shadow: 0 0 0 $border-width-focus $dark-theme-focus;
+			}
+		}
+	}
+
+	&.has-child-selected {
+		&::after {
+			box-shadow: 0 0 0 $border-width $gray-200;
+
+			.is-dark-theme & {
+				box-shadow: 0 0 0 $border-width-focus $gray-700;
+			}
+		}
+	}
+}