fix: improve substring ops with multi-byte chars

[reubeno]

There is more to understand with respect to differences from bash's output, but with this change brush no longer generates an error when trying to slice into a string somewhere other than at a character boundary.

[github-actions]

Test Results

    2 files      9 suites   1m 16s ⏱️
  525 tests   525 ✅ 0 💤 0 ❌
1 038 runs  1 038 ✅ 0 💤 0 ❌

Results for commit 57fb43c.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.43 μs	3.44 μs	0.00 μs	⚪ Unchanged
instantiate_shell	61.49 μs	60.75 μs	-0.74 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	30387.03 μs	30687.95 μs	300.92 μs	⚪ Unchanged
parse_bash_completion	2864.81 μs	2795.01 μs	-69.79 μs	🟢 -2.44%
parse_sample_script	4.26 μs	4.18 μs	-0.09 μs	⚪ Unchanged
run_echo_builtin_command	90.92 μs	90.82 μs	-0.10 μs	⚪ Unchanged
run_one_builtin_command	107.97 μs	109.93 μs	1.96 μs	⚪ Unchanged
run_one_external_command	2016.01 μs	1946.68 μs	-69.33 μs	🟢 -3.44%
run_one_external_command_directly	1007.92 μs	1007.36 μs	-0.56 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/expansion.rs	🟢 96.53%	🟢 96.26%	🔴 -0.27%
Overall Coverage	🟢 74.57%	🟢 74.58%	🟢 0.01%

Minimum allowed coverage is 70%, this run produced 74.58%