feat: add Koy syntax highlighting with StreamLanguage

[simonNozaki]

Summary

• @codemirror/language の StreamLanguage を使って Koy 言語のシンタックスハイライトを実装
• src/koy-language.ts にトークナイザーとハイライトスタイルを定義
  • キーワード（val fn if など）: #c586c0（紫）
  • リテラル（true false nil）: #569cd6（青）
  • ビルトイン（println）: #dcdcaa（黄）
  • 文字列: #ce9178（橙）
  • 数値: #b5cea8（緑）
  • コメント（//）: #6a9955（グレー、イタリック）
  • 演算子: #d4d4d4（白）
• ADR-0001 に StreamLanguage 採用の意思決定と Lezer パーサーを不採用とした背景を追記

Test plan

• エディタ上でキーワード・リテラル・文字列・数値・コメントに色がつくことを確認

🤖 Generated with Claude Code

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Test Coverage Report

Overall Project	91.38%	🍏

There is no coverage information present for the Files changed