feat: 式に符号を使えるように

[takejohn]

What

単項演算子の正号 +・負号 -が数値リテラル以外の式にも使用できるようになります。
正号のノードはPlus、負号のノードはMinusになります。
符号の付いた数値リテラルは従来通り単一の数値リテラルとして扱います。

Why

Resolve #43

Additional info (optional)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 78.78788% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/interpreter/index.ts	68.18%	7 Missing ⚠️

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Files with missing lines	Coverage Δ
src/node.ts	85.71% <ø> (-12.66%)	⬇️
src/parser/syntaxes/expressions.ts	95.18% <100.00%> (+5.99%)	⬆️
src/parser/visit.ts	100.00% <100.00%> (+5.26%)	⬆️
src/interpreter/index.ts	95.07% <68.18%> (+9.27%)	⬆️

... and 26 files with indirect coverage changes