fix: 複合代入文の左辺が1回だけ評価されるように

[takejohn]

What

複合代入文(+=, -=)の左辺が1回だけ評価されるようになります。
例:

var index = -1
let array = [0, 0]

@next() {
    index += 1
    return index
}

array[next()] += 1

<: array.map(Core:to_str).join(', ') // => '1, 0'

Why

Resolve #842

Additional info (optional)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 96.03960% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/interpreter/reference.ts	94.93%	4 Missing ⚠️

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Files with missing lines	Coverage Δ
src/interpreter/index.ts	94.07% <100.00%> (+8.27%)	⬆️
src/interpreter/reference.ts	94.93% <94.93%> (ø)

... and 29 files with indirect coverage changes