fix: address issues with array unset

[reubeno]

Also address parse issues with complex array index expressions.

[github-actions]

Test Results

  2 files    9 suites   1m 14s ⏱️
474 tests 474 ✅ 0 💤 0 ❌
939 runs  939 ✅ 0 💤 0 ❌

Results for commit dadee77.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.41 μs	3.42 μs	0.00 μs	⚪ Unchanged
instantiate_shell	60.41 μs	59.71 μs	-0.70 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	27814.58 μs	28622.15 μs	807.57 μs	🟠 +2.90%
parse_bash_completion	2642.06 μs	2650.22 μs	8.16 μs	⚪ Unchanged
parse_sample_script	3.91 μs	3.91 μs	0.00 μs	⚪ Unchanged
run_echo_builtin_command	90.84 μs	90.36 μs	-0.48 μs	⚪ Unchanged
run_one_builtin_command	108.24 μs	109.63 μs	1.39 μs	⚪ Unchanged
run_one_external_command	2003.25 μs	1931.97 μs	-71.29 μs	🟢 -3.56%
run_one_external_command_directly	1013.04 μs	1012.66 μs	-0.38 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/unset.rs	🟢 82.61%	🟢 82.98%	🟢 0.37%
brush-core/src/jobs.rs	🔴 37.23%	🔴 42.42%	🟢 5.19%
brush-core/src/shell.rs	🟢 82.55%	🟢 83.4%	🟢 0.85%
Overall Coverage	🟢 75.44%	🟢 75.57%	🟢 0.13%

Minimum allowed coverage is 70%, this run produced 75.57%