sort groups under files

Sources/XcodeGenKit/XCProjExtensions.swift

@@ -9,9 +9,9 @@ extension PBXFileElement {
 
     public var sortOrder: Int {
         if type(of: self).isa == "PBXGroup" {
-            return 0
-        } else {
             return 1
+        } else {
+            return 0
         }
     }
 }

Tests/Fixtures/TestProject/Project.xcodeproj/project.pbxproj

@@ -551,6 +551,11 @@
 		G_8448771205358 = {
 			isa = PBXGroup;
 			children = (
+				FR_479281060337 /* Folder */,
+				FR_815403394914 /* Headers */,
+				FR_232605427418 /* Mintfile */,
+				FR_257073931060 /* ResourceFolder */,
+				FR_775316160345 /* SomeFile */,
 				G_8252321105004 /* App */,
 				G_7831228999101 /* App_iOS_Tests */,
 				G_1235039993875 /* App_iOS_UITests */,
@@ -564,11 +569,6 @@
 				G_7189434949822 /* Resources */,
 				G_6651250437419 /* StandaloneFiles */,
 				G_3997550084026 /* StaticLibrary_ObjC */,
-				FR_479281060337 /* Folder */,
-				FR_815403394914 /* Headers */,
-				FR_232605427418 /* Mintfile */,
-				FR_257073931060 /* ResourceFolder */,
-				FR_775316160345 /* SomeFile */,
 				G_1952740716080 /* Frameworks */,
 				G_8620238527590 /* Products */,
 			);

Tests/XcodeGenKitTests/SourceGeneratorTests.swift

@@ -501,13 +501,13 @@ class SourceGeneratorTests: XCTestCase {
                 let group = pbxProj.objects.group(named: "Sources", inGroup: try pbxProj.getMainGroup())!.object
                 let names = group.children.compactMap { pbxProj.objects.getFileElement(reference: $0)?.nameOrPath }
                 try expect(names) == [
-                    "group",
-                    "group2",
                     "1file.a",
                     "10file.a",
                     "file.swift",
                     "file2.swift",
                     "file3.swift",
+                    "group",
+                    "group2",
                 ]
             }
         }