os: add windows cp

vlib/os/os.v

@@ -71,6 +71,8 @@ fn C.ftell(fp voidptr) int
 fn C.getenv(byteptr) byteptr
 fn C.sigaction(int, voidptr, int)
 
+fn C.GetLastError() u32
+
 // read_bytes reads an amount of bytes from the beginning of the file
 pub fn (f File) read_bytes(size int) []byte {
 	return f.read_bytes_at(size, 0)
@@ -126,13 +128,25 @@ pub fn mv(old, new string) {
 	}
 }
 
-// TODO implement actual cp()
-pub fn cp(old, new string) {
+fn C.CopyFile(&u32, &u32, int) int
+
+// TODO implement actual cp for linux
+pub fn cp(old, new string) ?bool {
 	$if windows {
-		panic('not implemented')
-	}	$else {
+		_old := old.replace('/', '\\')
+		_new := new.replace('/', '\\')
+		C.CopyFile(_old.to_wide(), _new.to_wide(), false)
+
+		result := C.GetLastError()
+		if result == 0 {
+			return true
+		} else {
+			return error_with_code('failed to copy $old to $new', int(result))
+		}
+	} $else {
 		os.system('cp $old $new')
-	}	
+		return true // TODO make it return true or error when cp for linux is implemented
+	}
 }
 
 fn vfopen(path, mode string) *C.FILE {

vlib/os/os_test.v

@@ -113,6 +113,23 @@ fn test_walk() {
 	os.rmdir(folder)
 }
 
+fn test_cp() {
+  $if windows {
+    old_file_name := './example.txt'
+    new_file_name := './new_example.txt'
+    
+    os.write_file(old_file_name, 'Test data 1 2 3, V is awesome #$%^[]!~⭐')
+    result := os.cp(old_file_name, new_file_name) or { panic('$err: errcode: $errcode') }
+
+    old_file := os.read_file(old_file_name) or { panic(err) }
+    new_file := os.read_file(new_file_name) or { panic(err) }
+    assert old_file == new_file
+    
+    os.rm(old_file_name)
+    os.rm(new_file_name)
+  }
+}
+
 //fn test_fork() {
 //  pid := os.fork()
 //  if pid == 0 {

vlib/os/os_windows.v

@@ -61,7 +61,7 @@ pub fn ls(path string) ?[]string {
 	// }
 	// C.FindClose(h_find_dir)
 	if !dir_exists(path) {
-		return error('ls() couldnt open dir "$path"')
+		return error('ls() couldnt open dir "$path": directory does not exist')
 	}
 	// NOTE: Should eventually have path struct & os dependant path seperator (eg os.PATH_SEPERATOR)
 	// we need to add files to path eg. c:\windows\*.dll or :\windows\*