[Bridge] Support nullability annotations in bridged methods

Summary:
Fixes a crash due to the selector regex not knowing about the nullability annotations. Adds support for both the core annotations `__nullable` and `__nonnull` plus their shorthand counterparts `nullable` and `nonnull`.

Objective-C allows the shorthand versions only at the front of a parameter type declaration like `(nullable NSString *)` but the regex will pick up `(NSString * nullable)` too. This shouldn't cause any adverse effects and I left the code this way to keep the regex readable.

Fixes #1795

Closes https://github.com/facebook/react-native/pull/1796
Github Author: James Ide <ide@jameside.com>

Test Plan:
 Wrote a bridge method that uses a nullability annotation and verified that it didn't cause the app to crash:
```
RCT_EXPORT_METHOD(method:(nullable NSNumber *)reactTag)
{
}
```

Also added a nullable annotation to RCTTest.

Libraries/RCTTest/RCTTestModule.m

@@ -55,7 +55,7 @@ RCT_EXPORT_METHOD(verifySnapshot:(RCTResponseSenderBlock)callback)
   }];
 }
 
-RCT_EXPORT_METHOD(sendAppEvent:(NSString *)name body:(id)body)
+RCT_EXPORT_METHOD(sendAppEvent:(NSString *)name body:(nullable id)body)
 {
   [_bridge.eventDispatcher sendAppEventWithName:name body:body];
 }

React/Base/RCTModuleMethod.m

@@ -40,10 +40,12 @@
     static NSRegularExpression *typeRegex;
     static NSRegularExpression *selectorRegex;
     if (!typeRegex) {
-      NSString *unusedPattern = @"(?:(?:__unused|__attribute__\\(\\(unused\\)\\)))";
+      NSString *unusedPattern = @"(?:__unused|__attribute__\\(\\(unused\\)\\))";
       NSString *constPattern = @"(?:const)";
-      NSString *constUnusedPattern = [NSString stringWithFormat:@"(?:(?:%@|%@)\\s*)", unusedPattern, constPattern];
-      NSString *pattern = [NSString stringWithFormat:@"\\(%1$@?(\\w+?)(?:\\s*\\*)?%1$@?\\)", constUnusedPattern];
+      NSString *nullabilityPattern = @"(?:__nullable|__nonnull|nullable|nonnull)";
+      NSString *annotationPattern = [NSString stringWithFormat:@"(?:(?:%@|%@|%@)\\s*)",
+                                     unusedPattern, constPattern, nullabilityPattern];
+      NSString *pattern = [NSString stringWithFormat:@"\\(%1$@?(\\w+?)(?:\\s*\\*)?%1$@?\\)", annotationPattern];
       typeRegex = [[NSRegularExpression alloc] initWithPattern:pattern options:0 error:NULL];
 
       selectorRegex = [[NSRegularExpression alloc] initWithPattern:@"(?<=:).*?(?=[a-zA-Z_]+:|$)" options:0 error:NULL];