fix(websub): validate callback URL (HTTPS), fix domain regex, enforce secret length

tsconfig.json

@@ -1,11 +1,11 @@
 {
   "compilerOptions": {
-    "target": "ES2020",
+    "target": "ES2021",
     "module": "ESNext",
     "moduleResolution": "node",
     "esModuleInterop": true,
     "strict": true,
-    "lib": ["ES2020"],
+    "lib": ["ES2021"],
     "types": ["@cloudflare/workers-types"],
     "outDir": "dist",
     "noEmit": true,