feat(Shell): add methods to add environmental variables and builtins

[Elsie19]

This will be specifically useful for my cases, where I will be inserting variables and builtins (https://github.com/pacstall/pacstall/wiki/101.1-Variables#built-in-variables) into the environment before I source/run a script, but I'm sure a lot of people wanting to interface with brush will also find it useful.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	18.35 μs	18.29 μs	-0.05 μs	⚪ Unchanged
eval_arithmetic	0.17 μs	0.17 μs	0.00 μs	⚪ Unchanged
expand_one_string	1.81 μs	1.83 μs	0.02 μs	⚪ Unchanged
for_loop	22.32 μs	22.42 μs	0.10 μs	⚪ Unchanged
function_call	2.35 μs	2.49 μs	0.14 μs	⚪ Unchanged
instantiate_shell	56.87 μs	56.99 μs	0.12 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	23402.66 μs	25633.90 μs	2231.24 μs	🟠 +9.53%
parse_bash_completion	1657.43 μs	1664.22 μs	6.79 μs	⚪ Unchanged
parse_sample_script	1.75 μs	1.72 μs	-0.03 μs	⚪ Unchanged
run_echo_builtin_command	15.75 μs	15.88 μs	0.12 μs	⚪ Unchanged
run_one_external_command	4399.59 μs	2496.26 μs	-1903.33 μs	🟢 -43.26%
tokenize_sample_script	2.82 μs	2.80 μs	-0.02 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/shell.rs	🟢 86.07%	🟢 85.91%	🔴 -0.16%
Overall Coverage	🟢 78.35%	🟢 78.34%	🔴 -0.01%

Minimum allowed coverage is 70%, this run produced 78.34%

Test Summary: bash-completion test suite

Outcome	Count	Percentage
✅ Pass	1305	61.88
❗️ Error	240	11.38
❌ Fail	192	9.10
⏩ Skip	357	16.93
❎ Expected Fail	15	0.71
📊 Total	2109	100.00