This project aims to create a POSIX-compliant shell, as specified in:
https://pubs.opengroup.org/onlinepubs/9799919799/utilities/contents.html

This section explains well the order of command execution as well as builtin
command names. Unspecified utilities may or may not be implemented.
https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_09_01_04

## Special Built-In Utilities

- [ ] break
- [ ] colon
- [ ] continue
- [ ] dot
- [ ] eval
- [x] exec
- [x] exit
- [ ] export
- [ ] readonly
- [ ] return
- [ ] set
- [ ] shift
- [ ] times
- [ ] trap
- [ ] unset

## Built-In Utilities
- [ ] alias
- [ ] bg
- [ ] cd
- [ ] command
- [x] echo
- [x] false
- [ ] fc
- [ ] fg
- [ ] getopts
- [ ] hash
- [ ] jobs
- [ ] kill
- [ ] printf
- [ ] pwd
- [ ] read
- [ ] test
- [x] true
- [ ] type
- [ ] ulimit
- [ ] umask
- [ ] unalias
- [ ] wait