BREAKING: Break TS build that uses functions.config() (#1757)

Type out `functions.config()` API. Use of `functions.config()` API should now cause a BUILD error at compile time.

```ts
import * as functions from "firebase-functions";
import {onRequest} from "firebase-functions/https";
import * as logger from "firebase-functions/logger";

const cfg = functions.config()

export const helloWorld = onRequest((request, response) => {
  logger.info("Hello logs!", cfg);
  response.send("Hello from Firebase!");
});
```
```bash
$ npm run build

> build
> tsc

src/index.ts:5:23 - error TS2349: This expression is not callable.
  Type 'never' has no call signatures.

5 const cfg = functions.config()
                        ~~~~~~


Found 1 error in src/index.ts:5
```

Author: taeold

spec/v1/config.spec.ts

@@ -26,11 +26,14 @@ import { config } from "../../src/v1/config";
 
 describe("config()", () => {
   it("throws an error with migration guidance", () => {
-    expect(config).to.throw(
+    expect(() => {
+      // @ts-expect-error - config is deprecated and typed as never to cause a build error
+      config();
+    }).to.throw(
       Error,
       "functions.config() has been removed in firebase-functions v7. " +
         "Migrate to environment parameters using the params module. " +
         "Migration guide: https://firebase.google.com/docs/functions/config-env#migrate-config"
     );
   });
-});
+});

src/v1/config.ts

@@ -27,10 +27,10 @@ export { firebaseConfig } from "../common/config";
  * Migrate to environment parameters using the `params` module immediately.
  * Migration guide: https://firebase.google.com/docs/functions/config-env#migrate-config
  */
-export function config(): Record<string, any> {
+export const config: never = (() => {
   throw new Error(
     "functions.config() has been removed in firebase-functions v7. " +
       "Migrate to environment parameters using the params module. " +
       "Migration guide: https://firebase.google.com/docs/functions/config-env#migrate-config"
   );
-}
+}) as never;