Fix the issue of "Initializers are not allowed in ambient contexts" for 3.0.x version of TS

lib/restError.ts

@@ -5,9 +5,9 @@ import { HttpOperationResponse } from "./httpOperationResponse";
 import { WebResource } from "./webResource";
 
 export class RestError extends Error {
-  static readonly REQUEST_SEND_ERROR = "REQUEST_SEND_ERROR";
-  static readonly REQUEST_ABORTED_ERROR = "REQUEST_ABORTED_ERROR";
-  static readonly PARSE_ERROR = "PARSE_ERROR";
+  static readonly REQUEST_SEND_ERROR: string = "REQUEST_SEND_ERROR";
+  static readonly REQUEST_ABORTED_ERROR: string = "REQUEST_ABORTED_ERROR";
+  static readonly PARSE_ERROR: string = "PARSE_ERROR";
 
   code?: string;
   statusCode?: number;

package.json

@@ -114,7 +114,8 @@
     "publish-preview": "mocha --no-colors && shx rm -rf dist/test && node ./.scripts/publish",
     "local": "ts-node ./.scripts/local.ts",
     "preview": "ts-node ./.scripts/preview.ts",
-    "latest": "ts-node ./.scripts/latest.ts"
+    "latest": "ts-node ./.scripts/latest.ts",
+    "prepack": "npm i && npm run build"
   },
   "sideEffects": false
 }