fix build error due to missing babel plugins (opensearch-project#661)

Signed-off-by: Yulong Ruan <ruanyl@amazon.com>

Author: ruanyl

babel.config.js

@@ -16,8 +16,8 @@ module.exports = {
     require('@babel/preset-typescript'),
   ],
   plugins: [
-    require('@babel/plugin-proposal-class-properties'),
-    require('@babel/plugin-proposal-object-rest-spread'),
+    require('@babel/plugin-transform-class-properties'),
+    require('@babel/plugin-transform-object-rest-spread'),
     ['@babel/plugin-transform-modules-commonjs', { allowTopLevelThis: true }],
     [require('@babel/plugin-transform-runtime'), { regenerator: true }],
   ],