chore(signature-v3-crt): add downlevel script (#2745)

Author: AllanZhengYP

packages/signature-v4-crt/package.json

@@ -9,7 +9,7 @@
     "build:cjs": "tsc -p tsconfig.cjs.json",
     "build:es": "tsc -p tsconfig.es.json",
     "build": "yarn build:es && yarn build:cjs",
-    "prepublishOnly": "yarn build",
+    "downlevel-dts": "downlevel-dts dist/types dist/types/ts3.4",
     "test": "jest --coverage"
   },
   "author": {