feat(builtins): implement `ls -F` (classify) option

[chaliy]

Problem

ls -F / ls --classify appends indicator characters to filenames based on type:

• / for directories
• * for executables
• @ for symlinks
• | for FIFOs
• = for sockets

This is a commonly used flag for quick visual file type identification.

References

• Inspired by just-bash #117
• POSIX ls: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ls.html