Annotate Module explicitly as @FunctionalInterface.

`Module` is implicitly a `@FunctionalInterface` already. That is very convenient, as it allows to express

```
methodThatTakesAModule(
    new AbstractModule(){
      @OverRide
      protected void configure() {
        bind(Foo.class).annotatedWith(Bar.class).toInstance(barredFoo);
      }
    }
);
```

as

```
methodThatTakesAModule(binder -> binder.bind(Foo.class).annotatedWith(Bar.class).toInstance(barredFoo));
```

PiperOrigin-RevId: 544017353

core/src/com/google/inject/Module.java

@@ -28,6 +28,7 @@
  * all methods annotated with {@literal @}{@link Provides}. Use scope and binding annotations on
  * these methods to configure the bindings.
  */
+@FunctionalInterface
 public interface Module {
 
   /**