fix: parse '#' as char only if inside a variable expansion

[idealseal]

Only in arithmetic expressions '#' is a valid token, in other contexts it is treated as a comment, and all following characters will be ignored by the tokenizer.

Closes: #408

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	18.52 μs	17.77 μs	-0.76 μs	🟢 -4.08%
eval_arithmetic	0.17 μs	0.17 μs	0.00 μs	⚪ Unchanged
expand_one_string	1.79 μs	1.82 μs	0.03 μs	⚪ Unchanged
for_loop	22.18 μs	21.95 μs	-0.23 μs	⚪ Unchanged
function_call	2.32 μs	2.33 μs	0.01 μs	⚪ Unchanged
instantiate_shell	56.96 μs	56.72 μs	-0.24 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	23729.14 μs	22736.16 μs	-992.97 μs	⚪ Unchanged
parse_bash_completion	1660.71 μs	1652.97 μs	-7.74 μs	⚪ Unchanged
parse_sample_script	1.76 μs	1.74 μs	-0.02 μs	🟢 -1.36%
run_echo_builtin_command	15.62 μs	15.32 μs	-0.30 μs	⚪ Unchanged
run_one_external_command	2035.24 μs	3881.02 μs	1845.78 μs	🟠 +90.69%
tokenize_sample_script	2.79 μs	2.84 μs	0.05 μs	🟠 +1.83%

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
Overall Coverage	🟢 78.28%	🟢 78.28%	⚪ 0%

Minimum allowed coverage is 70%, this run produced 78.28%

Test Summary: bash-completion test suite

Outcome	Count	Percentage
✅ Pass	1345	63.77
❗️ Error	229	10.86
❌ Fail	157	7.44
⏩ Skip	366	17.35
❎ Expected Fail	11	0.52
✔️ Unexpected Pass	1	0.05
📊 Total	2109	100.00

[reubeno]

Thanks for contributing a fix for the issue! I have a few comments/questions about testing, but the fix seems like a reasonable, targeted one to go with.

[reubeno]

Thanks for making those changes, @idealseal. LGTM