fix(Swift): Nullability stuff broke Swift compatibility (#1521)

CDVPlugin's commandDelegate is a weak pointer, which means technically
in Swift it should be an optional type that requires unwrapping. For
some reason, it is not.

If the wrap the CDVPlugin class in the ASSUME_NONNULL macro, Swift
suddenly starts enforcing that it's an optional, and this breaks all
existing Swift plugins.

You aren't allowed to combine `weak` and `nonnull`, and all the
properties in CDVPlugin are weak, so just... don't wrap it in
ASSUME_NONNULL to make life easier for everyone 🙃

CordovaLib/include/Cordova/CDVPlugin.h

@@ -32,6 +32,8 @@
 
 typedef int CDVWebViewNavigationType;
 
+NS_ASSUME_NONNULL_BEGIN
+
 #ifndef __swift__
 // This global extension to the UIView class causes issues for Swift subclasses
 // of UIView with their own scrollView properties, so we're removing it from
@@ -42,8 +44,6 @@ typedef int CDVWebViewNavigationType;
 @end
 #endif
 
-NS_ASSUME_NONNULL_BEGIN
-
 extern const NSNotificationName CDVPageDidLoadNotification;
 extern const NSNotificationName CDVPluginHandleOpenURLNotification;
 extern const NSNotificationName CDVPluginHandleOpenURLWithAppSourceAndAnnotationNotification CDV_DEPRECATED(8, "Find sourceApplication and annotations in the userInfo of the CDVPluginHandleOpenURLNotification notification.");
@@ -56,6 +56,8 @@ extern const NSNotificationName CDVViewWillLayoutSubviewsNotification;
 extern const NSNotificationName CDVViewDidLayoutSubviewsNotification;
 extern const NSNotificationName CDVViewWillTransitionToSizeNotification;
 
+NS_ASSUME_NONNULL_END
+
 @interface CDVPlugin : NSObject {}
 
 @property (nonatomic, readonly, weak) UIView* webView;
@@ -68,8 +70,8 @@ extern const NSNotificationName CDVViewWillTransitionToSizeNotification;
 
 - (void)pluginInitialize;
 
-- (void)handleOpenURL:(NSNotification*)notification;
-- (void)handleOpenURLWithApplicationSourceAndAnnotation:(NSNotification*)notification CDV_DEPRECATED(8, "Use the handleOpenUrl method and the notification userInfo data.");
+- (void)handleOpenURL:(nonnull NSNotification*)notification;
+- (void)handleOpenURLWithApplicationSourceAndAnnotation:(nonnull NSNotification*)notification CDV_DEPRECATED(8, "Use the handleOpenUrl method and the notification userInfo data.");
 - (void)onAppTerminate;
 - (void)onMemoryWarning;
 - (void)onReset;
@@ -83,12 +85,14 @@ extern const NSNotificationName CDVViewWillTransitionToSizeNotification;
  - (void) onOrientationDidChange {}
  */
 
-- (id)appDelegate;
+- (nonnull id)appDelegate;
 
 @end
 
 #pragma mark - Plugin protocols
 
+NS_ASSUME_NONNULL_BEGIN
+
 /**
  A protocol for Cordova plugins to intercept and respond to server
  authentication challenges through WebKit.