feat: Add buggy String Calculator implementation

- Implements basic string calculator with 5 intentional bugs
- Bug 1: Empty string returns 1 instead of 0
- Bug 2: Single number has off-by-one error
- Bug 3: Summation loop misses last element
- Bug 4: Newline delimiter not properly handled
- Bug 5: Custom delimiter parsing broken
- Ready for Bug Hunt Kata with TDD approach

bin/tdd_katas.dart

@@ -1,5 +1,3 @@
-import 'package:tdd_katas/roman_numerals.dart' as roman_numerals;
-
 void main(List<String> arguments) {
-  print('Hello world: ${roman_numerals.integerToRoman(1)}');
+  print('TDD Katas exercises, please read the README.md file.');
 }