refactor: prevent reinitializing global.$RefreshReg$ (#1510)

Summary:
Part of #1480

This PR adds a check for global.$RefreshReg$ to prevent reinitialising it in case there are multiple module systems present in runtime (like it does in the implementation of module-federation in Metro).

For context: for the HMR in MF we're reusing the global instance of `ReactRefresh`

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Changelog: [Internal] Prevent reinitialising global.$RefreshReg$ in require.js polyfill

Pull Request resolved: #1510

Test Plan: n/a

Reviewed By: huntie

Differential Revision: D75678288

Pulled By: robhogan

fbshipit-source-id: 2f7eb8ac3b298494b260276b1ab74e35230e3743

Author: jbroma

packages/metro-runtime/src/polyfills/require.js

@@ -94,8 +94,8 @@ const CYCLE_DETECTED = {};
 const {hasOwnProperty} = {};
 
 if (__DEV__) {
-  global.$RefreshReg$ = () => {};
-  global.$RefreshSig$ = () => type => type;
+  global.$RefreshReg$ = global.$RefreshReg$ ?? (() => {});
+  global.$RefreshSig$ = global.$RefreshSig$ ?? (() => type => type);
 }
 
 function clear(): ModuleList {