Implement proper error handling, add godoc comments, and implement expressions

cmd/scpt/main.go

@@ -23,5 +23,8 @@ func main() {
 	if err != nil {
 		log.Fatalln("Error parsing file:", err)
 	}
-	ast.Execute()
-}
+	err = ast.Execute()
+	if err != nil {
+		log.Fatalln("Error executing script:", err)
+	}
+}