Expose [RCTBundleURLProvider isPackagerRunning] publicly (facebook#27012)

Summary:
The private API isPackagerRunning is useful. Rather than duplicating it, I am exposing it here so that I can make use of it in future PRs, and so others can do the same.

## Changelog

[iOS] [Changed] - Expose the isPackagerRunning methods on RCTBundleURLProvider
Pull Request resolved: facebook#27012

Test Plan: This change doesn't impact any runtime code, though it does impact build. I used a vanilla test app (react-native init) and built it using xcode as well as react-native run-ios.

Differential Revision: D18174316

Pulled By: cpojer

fbshipit-source-id: 523d134882303f68a1f69521e31f29d7dbfb666c

Author: afoxman

React/Base/RCTBundleURLProvider.h

@@ -31,6 +31,10 @@ extern const NSUInteger kRCTBundleURLProviderDefaultPort;
  */
 - (void)resetToDefaults;
 
+#if RCT_DEV
+- (BOOL)isPackagerRunning:(NSString *)host;
+#endif
+
 /**
  * Returns the jsBundleURL for a given bundle entrypoint and
  * the fallback offline JS bundle if the packager is not running.