replace method
Replaces the content of the target template with a new value for this
entry. This knows it's place within the target and will replace the value
in the target with the newValue that was passed in.
Implementation
String replace(String target, String newValue) {
final prefix = target.substring(0, startPosition);
final suffix = target.substring(
startPosition + _value.length,
target.length,
);
return '$prefix$newValue$suffix';
}