fix(exit): correct exit semantics in various compund statements

[reubeno]

There were a handful of cases where exit was not being honored; this attempts to resolve the lot of them, as well as add a handful of test cases to confirm the behavior matches that of bash.

Notably, we need to ignore exit-shell requests from subshells, but need to honor them when coming from within the same shell.

(Note: the way in which we have separate representations for command spawn results, execution results, and builtin execution results... leaves room for improvement. The present goal is to reach correct behavior and then we should, in time, strive to simplify the logic.)
 
Resolves #337

[Copilot]

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	16.29 μs	16.21 μs	-0.08 μs	🟢 -0.51%
eval_arithmetic	0.18 μs	0.19 μs	0.01 μs	⚪ Unchanged
expand_one_string	1.70 μs	1.67 μs	-0.03 μs	⚪ Unchanged
for_loop	32.38 μs	32.82 μs	0.44 μs	⚪ Unchanged
function_call	3.46 μs	3.40 μs	-0.06 μs	⚪ Unchanged
instantiate_shell	48.34 μs	48.89 μs	0.55 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	22130.35 μs	23366.18 μs	1235.83 μs	🟠 +5.58%
parse_bash_completion	1659.83 μs	1681.74 μs	21.90 μs	⚪ Unchanged
parse_sample_script	1.77 μs	1.77 μs	0.00 μs	⚪ Unchanged
run_echo_builtin_command	16.47 μs	16.29 μs	-0.18 μs	⚪ Unchanged
run_one_external_command	2292.20 μs	7456.53 μs	5164.34 μs	🟠 +225.30%
tokenize_sample_script	2.86 μs	2.85 μs	-0.00 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/set.rs	🟠 61.93%	🟠 63.07%	🟢 1.14%
brush-core/src/commands.rs	🟢 86.83%	🟢 86.9%	🟢 0.07%
brush-core/src/interp.rs	🟢 91.02%	🟢 91.25%	🟢 0.23%
brush-core/src/jobs.rs	🔴 43.1%	🔴 37.07%	🔴 -6.03%
brush-core/src/shell.rs	🟢 82.49%	🟢 82.2%	🔴 -0.29%
brush-interactive/src/interactive_shell.rs	🟠 73.42%	🟠 74.68%	🟢 1.26%
Overall Coverage	🟢 77.03%	🟢 76.97%	🔴 -0.06%

Minimum allowed coverage is 70%, this run produced 76.97%

[github-actions]

Test Results

    3 files     14 suites   5m 1s ⏱️
  612 tests   612 ✅ 0 💤 0 ❌
1 822 runs  1 822 ✅ 0 💤 0 ❌

Results for commit 62f8295.