Implement getConstants()

Summary:
Instead of assigning all the constants exported by a NativeModule to the native module JavaScript object itself, we want to instead export a `getConstants()` method that can be used to access native module constants. This change simplifies the API of native modules. Eventually, we'll remove the ability to access constants as native module object properties alltogether, but that's comes later.

**Note**: I didn't need to make any cpp changes because `JSIExecutor::NativeModuleProxy::get` calls `JSINativeModules::getModule` (here: https://goo.gl/QwPDWF), which eventually calls `JSINativeModules::createModule`, which uses `global.__fbGenNativeModule` (here: https://goo.gl/pSxMgE), which is just an alias to `genModule` in `NativeModules` (here: https://goo.gl/u2wjCs).

Reviewed By: fkgozali

Differential Revision: D13207152

fbshipit-source-id: 375aab1346232819187a5d5b272b33c55992346a

Libraries/BatchedBridge/NativeModules.js

@@ -60,8 +60,17 @@ function genModule(
       const methodType = isPromise ? 'promise' : isSync ? 'sync' : 'async';
       module[methodName] = genMethod(moduleID, methodID, methodType);
     });
+
   Object.assign(module, constants);
 
+  if (module.getConstants == null) {
+    module.getConstants = () => constants;
+  } else {
+    console.warn(
+      `Unable to define method 'getConstants()' on NativeModule '${moduleName}'. NativeModule '${moduleName}' already has a constant or method called 'getConstants'. Please remove it.`,
+    );
+  }
+
   if (__DEV__) {
     BatchedBridge.createDebugLookup(moduleID, moduleName, methods);
   }