refactor: handling functions for cross-platform compatibility (#166)

- Change the `rmcmd` and `mkdircmd` functions to accept an OS parameter
- Remove `command_windows.go` file
- Modify `removeDestFile` and `Exec` functions to use the OS parameter
- Add OS detection logic to `removeAllDestFile` and `Exec` functions
- Modify `TestRemoveDestFile` function to use the OS parameter

fix https://github.com/appleboy/drone-scp/pull/119

command.go

@@ -1,12 +1,22 @@
-//go:build !windows
-// +build !windows
-
 package main
 
-func rmcmd(target string) string {
-	return "rm -rf " + target
+func rmcmd(os, target string) string {
+	switch os {
+	case "windows":
+		return "DEL /F /S " + target
+	case "unix":
+		return "rm -rf " + target
+	}
+	return ""
 }
 
-func mkdircmd(target string) string {
-	return "mkdir -p " + target
+func mkdircmd(os, target string) string {
+	switch os {
+	case "windows":
+		return "if not exist " + target + " mkdir " + target
+	case "unix":
+		return "mkdir -p " + target
+	}
+
+	return ""
 }