Implement more of compgen

[reubeno]

Adds naive implementation for file, alias, builtin, and function. Future work should figure out how to reduce allocations in this code path, particularly for candidates that don't match filters.

[github-actions]

Test Results

344 tests   329 ✅  2m 19s ⏱️
 56 suites   15 💤
  8 files      0 ❌

Results for commit 757c21f.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (ns)	Test/PR (ns)	Delta (ns)	Delta %
expand_one_string	3318 ns	3317 ns	-1 ns	🟢 -0.03%
instantiate_shell	53074 ns	53084 ns	+10 ns	🟠 +0.02%
instantiate_shell_with_init_scripts	26636306 ns	47027422 ns	+20391116 ns	🟠 +76.55%
parse_bash_completion	6644209 ns	6662033 ns	+17824 ns	🟠 +0.27%
parse_sample_script	11072 ns	11047 ns	-25 ns	🟢 -0.23%
run_echo_builtin_command	92046 ns	92726 ns	+680 ns	🟠 +0.74%
run_one_builtin_command	108151 ns	110861 ns	+2710 ns	🟠 +2.51%
run_one_external_command	1759258 ns	1787044 ns	+27786 ns	🟠 +1.58%
run_one_external_command_directly	1022626 ns	1022512 ns	-114 ns	🟢 -0.01%

Code Coverage Report

Package	Base Coverage	New Coverage	Difference
cli.src	🟢 78.82%	🟠 73.95%	🔴 -4.87%
interactive-shell.src	🟢 75.88%	🟢 75.88%	⚪ 0%
parser.src	🟢 85.84%	🟢 85.86%	🟢 0.02%
shell.src	🟢 80.06%	🟢 79.93%	🔴 -0.13%
shell.src.builtins	🟠 63.98%	🟠 64.03%	🟢 0.05%
Overall Coverage	🟢 78.09%	🟢 77.99%	🔴 -0.1%

Minimum allowed coverage is 0%, this run produced 77.99%