fix: handle default export in custom blocks (#1081)

Author: Jinjiang

lib/loader.js

@@ -380,8 +380,11 @@ module.exports = function (content) {
 
         output +=
           'var customBlock = ' + requireString + '\n' +
-          'if (typeof customBlock === "function") {' +
-          '  customBlock(Component)' +
+          'if (customBlock && customBlock.__esModule) {\n' +
+          '  customBlock = customBlock.default\n' +
+          '}\n' +
+          'if (typeof customBlock === "function") {\n' +
+          '  customBlock(Component)\n' +
           '}\n'
       }
     })