feat: add server to exports

In order to support using `@google-cloud/functions-framework` in integration tests, add `./server` to exports, mapped to "./build/src/server.js"

Author: penx

package.json

@@ -10,7 +10,8 @@
   "types": "build/src/index.d.ts",
   "exports": {
     ".": "./build/src/index.js",
-    "./testing": "./build/src/testing.js"
+    "./testing": "./build/src/testing.js",
+    "./server": "./build/src/server.js"
   },
   "dependencies": {
     "@types/express": "4.17.13",