enhance: implement AiSON.stringify

[kakkokari-gtyih]

What

AiSONのstringifyを実装
AiScript AST→stringの機構はなく、その想定が現時点ではなさそうなので、AiSONクラスに直接実装した

Why

Fix #939

Additional info (optional)

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/parser/aison.ts	97.77%	1 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Files with missing lines	Coverage Δ
src/parser/aison.ts	98.18% <97.77%> (ø)

... and 30 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.