feat: add Platform.isWindows static method to check for Windows OS

Add a new `isWindows` static method to the `Platform` class that returns `true` when the host operating system name equals "Windows". This provides a less stringly-typed API compared to checking `Platform.name` directly. The implementation is a simple delegation to the existing `name` foreign method, comparing it against the "Windows" string. Includes a test that verifies `Platform.isWindows` is equivalent to `Platform.name == "Windows"`.

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"