fix(builtins): correct parsing of bind positional arg

[reubeno]

bind is still largely unimplemented, but this at least improves the error messages it emits when a caller provides a key sequence as a positional argument.

Found by #380

[github-actions]

Test Results

    2 files      9 suites   1m 46s ⏱️
  671 tests   671 ✅ 0 💤 0 ❌
1 328 runs  1 328 ✅ 0 💤 0 ❌

Results for commit 04628ec.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	18.01 μs	17.98 μs	-0.03 μs	⚪ Unchanged
eval_arithmetic	0.17 μs	0.17 μs	0.00 μs	⚪ Unchanged
expand_one_string	1.75 μs	1.76 μs	0.01 μs	⚪ Unchanged
for_loop	21.98 μs	21.91 μs	-0.08 μs	⚪ Unchanged
function_call	2.39 μs	2.38 μs	-0.01 μs	⚪ Unchanged
instantiate_shell	57.65 μs	57.30 μs	-0.35 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	21184.77 μs	21221.01 μs	36.25 μs	⚪ Unchanged
parse_bash_completion	1656.55 μs	1668.26 μs	11.71 μs	⚪ Unchanged
parse_sample_script	1.75 μs	1.75 μs	-0.01 μs	⚪ Unchanged
run_echo_builtin_command	15.26 μs	15.18 μs	-0.07 μs	⚪ Unchanged
run_one_external_command	2252.08 μs	2304.18 μs	52.10 μs	🟠 +2.31%
tokenize_sample_script	2.78 μs	2.81 μs	0.03 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/jobs.rs	🔴 43.1%	🔴 37.07%	🔴 -6.03%
Overall Coverage	🟢 77.99%	🟢 77.89%	🔴 -0.1%

Minimum allowed coverage is 70%, this run produced 77.89%

Test Summary: bash-completion test suite

Outcome	Count	Percentage
✅ Pass	1323	63.00
❗️ Error	218	10.38
❌ Fail	185	8.81
⏩ Skip	359	17.10
❎ Expected Fail	14	0.67
✔️ Unexpected Pass	1	0.05
📊 Total	2100	100.00