REST API: Add export rel link for active theme response.

The `targetHints.allow` value is used by the Site Editor to enable/disable the theme export feature.

Props mamaduka, wildworks.
Fixes #57379.

git-svn-id: https://develop.svn.wordpress.org/trunk@60984 602fd350-edb4-49c9-b593-d223f7449a82

Author: Mamaduka

src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php

@@ -414,6 +414,15 @@ protected function prepare_links( $theme ) {
 			);
 		}
 
+		if ( $theme->is_block_theme() && $this->is_same_theme( $theme, wp_get_theme() ) ) {
+			$links['https://api.w.org/export-theme'] = array(
+				'href'        => rest_url( 'wp-block-editor/v1/export' ),
+				'targetHints' => array(
+					'allow' => current_user_can( 'export' ) ? array( 'GET' ) : array(),
+				),
+			);
+		}
+
 		return $links;
 	}