chore: simplify true and false builtins

[reubeno]

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[github-actions]

Test Results

    2 files      9 suites   1m 31s ⏱️
  666 tests   666 ✅ 0 💤 0 ❌
1 318 runs  1 318 ✅ 0 💤 0 ❌

Results for commit 41eeed3.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	17.80 μs	18.56 μs	0.77 μs	🟠 +4.30%
eval_arithmetic	0.17 μs	0.17 μs	-0.00 μs	⚪ Unchanged
expand_one_string	1.58 μs	1.56 μs	-0.02 μs	⚪ Unchanged
for_loop	21.85 μs	21.68 μs	-0.17 μs	⚪ Unchanged
function_call	2.33 μs	2.37 μs	0.05 μs	⚪ Unchanged
instantiate_shell	57.65 μs	57.83 μs	0.18 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	21526.26 μs	21632.16 μs	105.90 μs	⚪ Unchanged
parse_bash_completion	1659.70 μs	1666.92 μs	7.23 μs	⚪ Unchanged
parse_sample_script	1.72 μs	1.74 μs	0.02 μs	🟠 +1.10%
run_echo_builtin_command	15.27 μs	15.72 μs	0.45 μs	⚪ Unchanged
run_one_external_command	2361.87 μs	2355.93 μs	-5.94 μs	⚪ Unchanged
tokenize_sample_script	2.94 μs	2.80 μs	-0.14 μs	🟢 -4.63%

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/false_.rs	🟢 100%	🔴 44.44%	🔴 -55.56%
brush-core/src/builtins/true_.rs	🟢 100%	🔴 44.44%	🔴 -55.56%
brush-core/src/interp.rs	🟢 91.46%	🟢 91.48%	🟢 0.02%
brush-core/src/jobs.rs	🔴 37.07%	🔴 43.1%	🟢 6.03%
Overall Coverage	🟢 77.81%	🟢 77.81%	⚪ 0%

Minimum allowed coverage is 70%, this run produced 77.81%