encoding.csv: add a new utility fn new_reader_from_file/2 (#20530)

vlib/encoding/csv/utils_test.v

@@ -0,0 +1,25 @@
+// Copyright (c) 2019-2024 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+module csv
+
+import os
+
+fn test_new_reader_from_file() {
+	test_file_path_for_reader := os.join_path(os.temp_dir(), 'test_new_reader_from_file.csv')
+
+	text := 'id,bonus,amount,yes\n1,bomb,1,true\n2,rocket,1,false,\n3,lightning,2,2\n'
+	os.write_file(test_file_path_for_reader, text)!
+
+	mut reader := new_reader_from_file(test_file_path_for_reader)!
+	mut writer := new_writer()
+
+	for {
+		row := reader.read() or { break }
+		writer.write(row) or { panic(err) }
+	}
+
+	assert text == writer.str()
+
+	os.rm(test_file_path_for_reader)!
+}