fix: honor IFS in read builtin

[reubeno]

Ensure that splitting happening in read honors the current effective value of IFS in the current environment.

Among other things, this fixes completion for arguments to tar. (The custom completion routines for tar in bash-completion do custom parsing of the --help output of tar; that parsing required reading in that output and using complex regexes to match against it.)

[github-actions[bot]]

Test Results

  2 files    9 suites   1m 7s :stopwatch: 470 tests 470 :white_check_mark: 0 :zzz: 0 :x: 931 runs  931 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 55fd9154.

[github-actions[bot]]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.40 μs	3.40 μs	0.00 μs	⚪ Unchanged
instantiate_shell	60.35 μs	59.91 μs	-0.44 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	28420.77 μs	28564.46 μs	143.69 μs	⚪ Unchanged
parse_bash_completion	2709.96 μs	2709.72 μs	-0.25 μs	⚪ Unchanged
parse_sample_script	4.01 μs	3.96 μs	-0.04 μs	⚪ Unchanged
run_echo_builtin_command	90.99 μs	89.70 μs	-1.29 μs	⚪ Unchanged
run_one_builtin_command	108.07 μs	107.00 μs	-1.06 μs	⚪ Unchanged
run_one_external_command	1972.55 μs	2037.43 μs	64.88 μs	🟠 +3.29%
run_one_external_command_directly	1012.10 μs	1015.45 μs	3.35 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/read.rs	🟠 65%	🟠 66.27%	🟢 1.27%
brush-core/src/shell.rs	🟢 76.97%	🟢 77.4%	🟢 0.43%
brush-shell/src/main.rs	🟢 89.61%	🟢 90.26%	🟢 0.65%
Overall Coverage	🟢 72.54%	🟢 72.58%	🟢 0.04%

Minimum allowed coverage is 70%, this run produced 72.58%