feat: 名前空間下の変数定義に属性を付与できるように

[takejohn]

What

名前空間下の変数定義に属性を付与できるようにします。

Why

• Resolve 名前空間下の変数定義の属性 #868

Additional info (optional)

[codecov-commenter]

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All modified and coverable lines are covered by tests ✅

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Files with missing lines	Coverage Δ
src/parser/syntaxes/statements.ts	97.37% <100.00%> (+4.12%)	⬆️
src/parser/syntaxes/toplevel.ts	96.15% <100.00%> (-2.41%)	⬇️

... and 28 files with indirect coverage changes

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