feat(runtime): add nullability annotations to NativeScript.h

Without them every pointer in the embedder API imports into Swift as
implicitly unwrapped, which hides the contract the implementation
already has: BaseDir, the script string and the Config argument are
required (their UTF8String feeds a std::string), while ApplicationPath,
MetadataPtr and Arguments are optional with defaults (BaseDir/app, the
__TNSMetadata section, no inspector arguments). initWithConfig: never
returns nil.

Author: adrian-niculescu

NativeScript/NativeScript.h

@@ -1,14 +1,16 @@
 #import <Foundation/Foundation.h>
 
+NS_ASSUME_NONNULL_BEGIN
+
 @interface Config : NSObject
 
 @property (nonatomic, retain) NSString* BaseDir;
-@property (nonatomic, retain) NSString* ApplicationPath;
-@property (nonatomic) void* MetadataPtr;
+@property (nonatomic, retain, nullable) NSString* ApplicationPath;
+@property (nonatomic, nullable) void* MetadataPtr;
 @property BOOL IsDebug;
 @property BOOL LogToSystemConsole;
 @property int ArgumentsCount;
-@property (nonatomic) char** Arguments;
+@property (nonatomic) char* _Nullable* _Nullable Arguments;
 
 @end
 
@@ -26,3 +28,5 @@
 - (bool)liveSync;
 
 @end
+
+NS_ASSUME_NONNULL_END