feat: #2 add pig latin detection

[dpuglielli]

Summary

• Extract core logic to pig-latin.js module with 4 exported functions
• Add isPigLatin(word) for single word detection (vowel-"way" and consonant-"ay" patterns)
• Add containsPigLatin(text) for text-level detection (returns true if ANY word matches)
• Update REPL to warn and skip conversion if input contains Pig Latin
• Add comprehensive test suite (26 tests)

Test plan

• Run npm test - all 26 tests pass
• Run npm start and test:
  • "hello world" converts to "ellohay orldway"
  • "ellohay orldway" shows "Input appears to be Pig Latin already"
  • "hello ellohay" (mixed) shows "Input appears to be Pig Latin already"

Closes #2