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)

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

... and 30 files with indirect coverage changes

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[syuilo]

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