[gym] Add support for multiplatform builds (#21319)

* Add multiplatform example project

* Add multiplatform? predicate to Project

* Detect when a multiplatform project is being built for a particular platform

* Rubocop fixes

* Update to latest version of xcodeproj

Ensure fastlane can parse Xcode 14 projects

fastlane_core/lib/fastlane_core/project.rb

@@ -307,6 +307,10 @@ def watchos?
       supported_platforms.include?(:watchOS)
     end
 
+    def multiplatform?
+      supported_platforms.count > 1
+    end
+
     def supported_platforms
       supported_platforms = build_settings(key: "SUPPORTED_PLATFORMS")
       if supported_platforms.nil?

fastlane_core/spec/project_spec.rb

@@ -209,6 +209,10 @@ def within_a_temp_dir
         expect(@project.ios?).to eq(true)
       end
 
+      it "#multiplatform?", requires_xcode: true do
+        expect(@project.multiplatform?).to eq(false)
+      end
+
       it "#tvos?", requires_xcode: true do
         expect(@project.tvos?).to eq(false)
       end
@@ -250,6 +254,10 @@ def within_a_temp_dir
         expect(@project.tvos?).to eq(false)
       end
 
+      it "#multiplatform?", requires_xcode: true do
+        expect(@project.multiplatform?).to eq(false)
+      end
+
       it "schemes", requires_xcodebuild: true do
         expect(@project.schemes).to eq(["Mac"])
       end
@@ -273,6 +281,10 @@ def within_a_temp_dir
         expect(@project.tvos?).to eq(true)
       end
 
+      it "#multiplatform?", requires_xcode: true do
+        expect(@project.multiplatform?).to eq(false)
+      end
+
       it "schemes", requires_xcodebuild: true do
         expect(@project.schemes).to eq(["ExampleTVOS"])
       end
@@ -300,6 +312,10 @@ def within_a_temp_dir
         expect(@project.tvos?).to eq(true)
       end
 
+      it "#multiplatform?", requires_xcode: true do
+        expect(@project.multiplatform?).to eq(true)
+      end
+
       it "schemes", requires_xcodebuild: true do
         expect(@project.schemes).to eq(["CrossPlatformFramework"])
       end