[macOS] Enable Impeller by default on macOS. (#164572)

Enables impeller by default on macOS devices. An opt out can still be
configured by passing --no-enable-impeller or using the
FLTEnableImpeller / NO setting in the Info.plist.

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject.mm

@@ -65,7 +65,7 @@ static NSString* const kAppBundleIdentifier = @"io.flutter.flutter.app";
   if (enableImpeller != nil) {
     return enableImpeller.boolValue;
   }
-  return NO;
+  return YES;
 }
 
 - (NSString*)assetsPath {