write method
Implementation
void write(String section, int lineNum, String line) {
final equalIndex = line.indexOf('=');
if (equalIndex == -1) {
throw Exception('parse the content error : line $lineNum');
}
final key = line.substring(0, equalIndex);
final value = line.substring(equalIndex + 1);
addConfig(section, key.trim(), value.trim());
}