Add direct kana conversion and utility functions

[manab-pr]

Summary

This PR introduces new features and improvements to enhance Japanese text processing capabilities in the library.

New Features

• Direct Kana Conversion

  • HiraganaToKatakana() – Convert hiragana to katakana
  • KatakanaToHiragana() – Convert katakana to hiragana

• Character Detection

  • ContainsKana() – Check if string contains kana
  • ContainsHiragana() – Check if string contains hiragana
  • ContainsKatakana() – Check if string contains katakana
  • ContainsKanji() – Check if string contains kanji

• Character Counting

  • CountKana() – Count all kana characters
  • CountHiragana() – Count hiragana characters
  • CountKatakana() – Count katakana characters
  • CountKanji() – Count kanji characters
  • CountRomaji() – Count romaji (Latin) characters

Improvements

• Added empty string handling for all conversion functions
• Introduced named constants for special kana (小さいつ, ん, ー)
• Refactored code to replace magic strings with constants
• Fixed typo in IsKanji documentation

Testing

• Added unit tests for all new functions
• Verified empty string edge cases
• Included benchmark tests for performance validation

Documentation

• Updated README.md with examples for new functions
• Added go.mod for Go module support

Test Plan

All tests pass successfully:

go test -v
go test -bench=.