Platform.isWindows.

src/module/os.wren

@@ -1,6 +1,8 @@
 class Platform {
   foreign static isPosix
   foreign static name
+
+  static isWindows { name == "Windows" }
 }
 
 class Process {

src/module/os.wren.inc

@@ -3,6 +3,8 @@ static const char* osModuleSource =
 "class Platform {\n"
 "  foreign static isPosix\n"
 "  foreign static name\n"
+"\n"
+"  static isWindows { name == \"Windows\" }\n"
 "}\n"
 "\n"
 "class Process {\n"