Use cfg aliases throught the code base

Co-authored-by: Mads Marquart <mads@marquart.dk>

src/platform_impl/windows/mod.rs

@@ -1,4 +1,4 @@
-#![cfg(target_os = "windows")]
+#![cfg(windows_platform)]
 
 use windows_sys::Win32::{
     Foundation::{HANDLE, HWND},

src/platform_impl/windows/window.rs

@@ -1,4 +1,4 @@
-#![cfg(target_os = "windows")]
+#![cfg(windows_platform)]
 
 use raw_window_handle::{
     RawDisplayHandle, RawWindowHandle, Win32WindowHandle, WindowsDisplayHandle,