Fusebox/RDT: setup global for fusebox runtime (facebook#43418)

Summary:
Pull Request resolved: facebook#43418

Changelog: [Internal]

This diff adds a script, which will be later imported from `InitializeCore` to setup a required global for communication between React Native runtime (RDT Backend in it) and Chrome DevTools Frontend (RDT Frontend in it).

See README for the architecture overview and how bidirectional communication is established.

Corresponding PR in Chrome DevTools frontend - facebookexperimental/rn-chrome-devtools-frontend#15

Differential Revision: D54770207

packages/react-native/Libraries/Core/fusebox/README.md

@@ -0,0 +1,12 @@
+# Fusebox Runtime
+
+## What is Fusebox?
+https://fburl.com/wut/j7u7kgal
+
+## Architecture
+
+### Frontend to Backend communication
+![Frontend to backend communication diagram](./assets/frontend-to-backend.excalidraw-embedded.png)
+
+### Backend to Frontend communication
+![Backend to frontend communication diagram](./assets/backend-to-frontend.excalidraw-embedded.png)

packages/react-native/Libraries/Core/fusebox/setUpFuseboxRuntime.js

@@ -0,0 +1,106 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+type JSONValue =
+  | string
+  | number
+  | boolean
+  | null
+  | {[key: string]: JSONValue}
+  | Array<JSONValue>;
+type DomainName = 'react-devtools';
+
+class EventScope<T> {
+  #cache: Set<(T) => void> = new Set();
+
+  addEventListener(listener: T => void): void {
+    this.#cache.add(listener);
+  }
+
+  removeEventListener(listener: T => void): void {
+    this.#cache.delete(listener);
+  }
+
+  // Should be hidden.
+  emit(value: T) {
+    for (const listener of this.#cache) {
+      listener(value);
+    }
+  }
+}
+
+class Domain {
+  name: DomainName;
+  onMessage: EventScope<JSONValue>;
+
+  constructor(name: DomainName) {
+    if (global[FuseboxRuntime.BINDING_NAME] == null) {
+      throw new Error(
+        `Could not create domain ${name}: receiving end doesn't exist`,
+      );
+    }
+
+    this.name = name;
+    this.onMessage = new EventScope<JSONValue>();
+  }
+
+  sendMessage(message: JSONValue) {
+    const messageWithDomain = {domain: this.name, message};
+    const serializedMessageWithDomain = JSON.stringify(messageWithDomain);
+
+    global[FuseboxRuntime.BINDING_NAME](serializedMessageWithDomain);
+  }
+}
+
+class FuseboxRuntime {
+  static #domainNameToDomainMap: Map<DomainName, Domain> = new Map();
+
+  // Referenced and initialized from Chrome DevTools frontend.
+  static BINDING_NAME: string = '__CHROME_DEVTOOLS_FRONTEND_BINDING__';
+  static onDomainInitialization: EventScope<Domain> = new EventScope<Domain>();
+
+  // Should be private, referenced from Chrome DevTools frontend.
+  static initializeDomain(domainName: DomainName): Domain {
+    const domain = new Domain(domainName);
+
+    this.#domainNameToDomainMap.set(domainName, domain);
+    this.onDomainInitialization.emit(domain);
+
+    return domain;
+  }
+
+  // Should be private, referenced from Chrome DevTools frontend.
+  static sendMessage(domainName: DomainName, message: string): void {
+    const domain = this.#domainNameToDomainMap.get(domainName);
+    if (domain == null) {
+      throw new Error(
+        `Could not send message to ${domainName}: domain doesn't exist`,
+      );
+    }
+
+    try {
+      const parsedMessage = JSON.parse(message);
+      domain.onMessage.emit(parsedMessage);
+    } catch (err) {
+      console.error(
+        `Error while trying to send a message to domain ${domainName}:`,
+        err,
+      );
+    }
+  }
+}
+
+Object.defineProperty(global, '__FUSEBOX_RUNTIME__', {
+  value: FuseboxRuntime,
+  configurable: false,
+  enumerable: false,
+  writable: false,
+});