Sign macOS project target for Development

This fixes a crash-on-launch bug when running Product > Run from Xcode (Product > Archive should work correctly). See sparkle-project/Sparkle#2056 for more details.

Bug introduced by commit:
671b054

WriteFreely-MultiPlatform.xcodeproj/project.pbxproj

@@ -1243,7 +1243,7 @@
 				ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon;
 				ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME = AccentColor;
 				CODE_SIGN_ENTITLEMENTS = macOS/macOS.entitlements;
-				CODE_SIGN_IDENTITY = "-";
+				CODE_SIGN_IDENTITY = "Apple Development";
 				CODE_SIGN_STYLE = Automatic;
 				COMBINE_HIDPI_IMAGES = YES;
 				CURRENT_PROJECT_VERSION = 620;
@@ -1270,7 +1270,7 @@
 				ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon;
 				ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME = AccentColor;
 				CODE_SIGN_ENTITLEMENTS = macOS/macOS.entitlements;
-				CODE_SIGN_IDENTITY = "-";
+				CODE_SIGN_IDENTITY = "Apple Development";
 				CODE_SIGN_STYLE = Automatic;
 				COMBINE_HIDPI_IMAGES = YES;
 				CURRENT_PROJECT_VERSION = 620;