addBefore method
Writes content
before tag
Implementation
void addBefore({required String tag, required String content}) {
final original = readAsStringSync();
final startIndex = original.indexOf(tag);
if (startIndex == -1) {
throw "startTag $tag not found in ${absolute.path}";
}
final mutated = original.replaceRange(startIndex, startIndex, content);
writeAsStringSync(mutated);
}