fix: correct parsing of parens in arithmetic command

[reubeno]

Among other things, fixes the case of (( (0) != 1 )).

[github-actions]

Test Results

    2 files      9 suites   1m 14s ⏱️
  517 tests   517 ✅ 0 💤 0 ❌
1 022 runs  1 022 ✅ 0 💤 0 ❌

Results for commit aaf5079.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.46 μs	3.46 μs	-0.00 μs	⚪ Unchanged
instantiate_shell	60.68 μs	60.63 μs	-0.05 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	30972.70 μs	32020.16 μs	1047.46 μs	🟠 +3.38%
parse_bash_completion	2821.97 μs	2836.22 μs	14.26 μs	⚪ Unchanged
parse_sample_script	4.37 μs	4.30 μs	-0.06 μs	🟢 -1.49%
run_echo_builtin_command	93.43 μs	91.90 μs	-1.53 μs	⚪ Unchanged
run_one_builtin_command	111.11 μs	110.54 μs	-0.58 μs	⚪ Unchanged
run_one_external_command	2063.89 μs	1942.35 μs	-121.54 μs	🟢 -5.89%
run_one_external_command_directly	1016.54 μs	1009.29 μs	-7.25 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/shell.rs	🟢 77.45%	🟢 78.01%	🟢 0.56%
brush-shell/src/main.rs	🟢 90.26%	🟢 90.91%	🟢 0.65%
Overall Coverage	🟢 74.34%	🟢 74.38%	🟢 0.04%

Minimum allowed coverage is 70%, this run produced 74.38%