• Initial release with string utility extension methods

Added #

• charCount(): Returns total character count in a string.
• charCountWithoutSpaces(): Returns count excluding whitespace.
• countUpperAndLowerCase(): Returns upper/lowercase character counts.
• removeDigits() to remove digits from string.
• isEmptyOrNull to check if a string is null or empty.
• toggle() extension to reverse boolean values.

Fixed #

• Applied Dart formatter to improve code style and pass pub.flutter-io.cn analysis.