chore: extract InteractiveShell as trait + refactor

[reubeno]

Extracts out InteractiveShell as a trait, albeit with a few default implementations. (That may be addressed in the future.)

This is intended to pave the way to allow for swapping things out for an alternate implementation experimentally.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.60 μs	3.61 μs	0.01 μs	⚪ Unchanged
instantiate_shell	59.38 μs	59.56 μs	0.18 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	27412.36 μs	27432.14 μs	19.78 μs	⚪ Unchanged
parse_bash_completion	5625.62 μs	5599.30 μs	-26.32 μs	⚪ Unchanged
parse_sample_script	8.85 μs	8.88 μs	0.03 μs	⚪ Unchanged
run_echo_builtin_command	92.80 μs	92.52 μs	-0.28 μs	⚪ Unchanged
run_one_builtin_command	112.20 μs	113.33 μs	1.13 μs	⚪ Unchanged
run_one_external_command	2003.51 μs	1927.08 μs	-76.43 μs	🟢 -3.81%
run_one_external_command_directly	998.98 μs	1004.07 μs	5.08 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/unalias.rs	🔴 0%	🟢 85%	🟢 85%
brush-core/src/jobs.rs	🔴 43.48%	🔴 41.3%	🔴 -2.18%
brush-core/src/shell.rs	🟢 78.01%	🟢 77.9%	🔴 -0.11%
brush-interactive/src/interactive_shell.rs	🟢 76.96%	🟠 66.1%	🔴 -10.86%
brush-interactive/src/rustyline_shell.rs	🔴 0%	🟢 81.87%	🟢 81.87%
brush-shell/src/main.rs	🟢 88.54%	🟢 89.6%	🟢 1.06%
brush-shell/src/shell_factory.rs	🔴 0%	🔴 13.04%	🟢 13.04%
Overall Coverage	🟢 76.29%	🟢 76.32%	🟢 0.03%

Minimum allowed coverage is 70%, this run produced 76.32%

[github-actions]

Test Results

410 tests   396 ✅  25s ⏱️
 68 suites   14 💤
 11 files      0 ❌

Results for commit fd57d6b.