Fix loading from Metro in Bridgeless mode (#29453)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/29453

Problem Statement: A native module needs to call a function on `ReactInstance` (in this case `loadScript`). Typically, this is handled by the bridge.
Current Bridgeless Solution: Create a new protocol (in this case `RCTJSScriptLoaderModule`) which lets a block be passed in TM init to forward the method call to `ReactInstance`. This is the best thing I could think of right now.

Changelog:[Internal]

Reviewed By: RSNara

Differential Revision: D22512748

fbshipit-source-id: e6559279b6e299e17d1199407129ad3902c41e6b

React/Base/RCTJSScriptLoaderModule.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+@class RCTSource;
+
+/**
+ * This protocol should be adopted when a turbo module needs to tell React Native to load a script.
+ * In bridge-less React Native, it is a replacement for [_bridge loadAndExecuteSplitBundleURL:].
+ */
+@protocol RCTJSScriptLoaderModule <NSObject>
+
+@property (nonatomic, copy, nonnull) void (^loadScript)(RCTSource *source);
+
+@end